Update slot mappings migration tool to change the training data version to 3.0
When a user migrates their training data from Rasa Open Source 2.0 to Rasa Open Source 3.0 in order to use the new slot mappings, the convertor should also change the training data version from 2.0 to 3.0
diff --git a/rasa/core/migrate.py b/rasa/core/migrate.py
index e536a8af6df..07530bb136a 100644
--- a/rasa/core/migrate.py
+++ b/rasa/core/migrate.py
@@ -160,12 +160,14 @@ def _assemble_new_domain(
     domain_file: Path, new_forms: Dict[Text, Any], new_slots: Dict[Text, Any]
 ) -> Dict[Text, Any]:
     original_content = rasa.shared.utils.io.read_yaml_file(domain_file)
-    new_domain = {}
+    new_domain: Dict[Text, Any] = {}
     for key, value in original_content.items():
         if key == KEY_SLOTS:
             new_domain.update({key: new_slots})
         elif key == KEY_FORMS:
             new_domain.update({key: new_forms})
+        elif key == "version":
+            new_domain.update({key: '"3.0"'})
         else:
             new_domain.update({key: value})
     return new_domain
@@ -198,6 +200,9 @@ def _migrate_domain_files(
         original_content = _create_back_up(file, backup)
 
         if KEY_SLOTS not in original_content and KEY_FORMS not in original_content:
+            if isinstance(original_content, dict):
+                original_content.update({"version": '"3.0"'})
+
             # this is done so that the other domain files can be moved
             # in the migrated directory
             rasa.shared.utils.io.write_yaml(
diff --git a/tests/core/test_migrate.py b/tests/core/test_migrate.py
index cf64c083311..b80cff9b6c9 100644
--- a/tests/core/test_migrate.py
+++ b/tests/core/test_migrate.py
@@ -30,7 +30,7 @@ def test_migrate_domain_format_with_required_slots(
     existing_domain_file = prepare_domain_path(
         tmp_path,
         """
-        version: "3.0"
+        version: "2.0"
         intents:
         - greet
         - affirm
@@ -77,6 +77,9 @@ def test_migrate_domain_format_with_required_slots(
 
     migrated_domain = rasa.shared.utils.io.read_yaml_file(domain_out_file)
 
+    migrated_training_data_version = migrated_domain.get("version")
+    assert migrated_training_data_version == '"3.0"'
+
     migrated_slots = migrated_domain.get("slots")
     expected_slots = {
         "location": {
@@ -133,7 +136,7 @@ def test_migrate_domain_form_without_required_slots(
     existing_domain_file = prepare_domain_path(
         tmp_path,
         """
-        version: "3.0"
+        version: "2.0"
         intents:
         - greet
         - affirm
@@ -247,7 +250,7 @@ def test_migrate_domain_with_diff_slot_types(
     existing_domain_file = prepare_domain_path(
         tmp_path,
         f"""
-        version: "3.0"
+        version: "2.0"
         entities:
             - outdoor
         slots:
@@ -299,7 +302,7 @@ def test_migrate_domain_format_from_dir(tmp_path: Path):
     prepare_domain_path(
         domain_dir,
         """
-        version: "3.0"
+        version: "2.0"
         entities:
             - outdoor
         slots:
@@ -313,7 +316,7 @@ def test_migrate_domain_format_from_dir(tmp_path: Path):
     prepare_domain_path(
         domain_dir,
         """
-        version: "3.0"
+        version: "2.0"
         forms:
           reservation_form:
             required_slots:
@@ -340,13 +343,17 @@ def test_migrate_domain_format_from_dir(tmp_path: Path):
 
     for file in domain_out_dir.iterdir():
         assert file.name in ["slots.yml", "forms.yml"]
+        migrated_file = rasa.shared.utils.io.read_yaml_file(file)
+
+        migrated_training_data_version = migrated_file.get("version")
+        assert migrated_training_data_version == '"3.0"'
 
 
 def test_migrate_domain_all_keys(tmp_path: Path, domain_out_file: Path):
     existing_domain_file = prepare_domain_path(
         tmp_path,
         """
-        version: "3.0"
+        version: "2.0"
         intents:
         - greet
         entities:
@@ -386,12 +393,15 @@ def test_migrate_domain_all_keys(tmp_path: Path, domain_out_file: Path):
     migrated_actions = migrated_domain.get("actions")
     assert "action_check_time" in migrated_actions
 
+    migrated_training_data_version = migrated_domain.get("version")
+    assert migrated_training_data_version == '"3.0"'
+
 
 def test_migrate_domain_format_with_custom_slot(tmp_path: Path, domain_out_file: Path):
     existing_domain_file = prepare_domain_path(
         tmp_path,
         """
-        version: "3.0"
+        version: "2.0"
         intents:
         - greet
         - affirm
@@ -447,7 +457,7 @@ def test_migrate_domain_with_no_requested_slot_for_from_entity_mappings(
     existing_domain_file = prepare_domain_path(
         tmp_path,
         """
-        version: "3.0"
+        version: "2.0"
         intents:
         - greet
         - affirm
@@ -499,7 +509,7 @@ def test_migrate_domain_format_duplicated_slots_in_forms(
     existing_domain_file = prepare_domain_path(
         tmp_path,
         """
-        version: "3.0"
+        version: "2.0"
         intents:
         - greet
         - affirm
@@ -592,7 +602,7 @@ def test_migrate_domain_dir_with_out_path_as_file(tmp_path: Path):
     prepare_domain_path(
         domain_dir,
         """
-        version: "3.0"
+        version: "2.0"
         entities:
             - outdoor
         slots:
@@ -606,7 +616,7 @@ def test_migrate_domain_dir_with_out_path_as_file(tmp_path: Path):
     prepare_domain_path(
         domain_dir,
         """
-        version: "3.0"
+        version: "2.0"
         forms:
           reservation_form:
             required_slots:
@@ -639,7 +649,7 @@ def test_migrate_domain_multiple_files_with_duplicate_slots(tmp_path: Path,):
     prepare_domain_path(
         domain_dir,
         """
-        version: "3.0"
+        version: "2.0"
         entities:
             - outdoor
         slots:
@@ -653,7 +663,7 @@ def test_migrate_domain_multiple_files_with_duplicate_slots(tmp_path: Path,):
     prepare_domain_path(
         domain_dir,
         """
-        version: "3.0"
+        version: "2.0"
         entities:
             - outdoor
         slots:
@@ -678,7 +688,7 @@ def test_migrate_domain_with_multiple_files_with_duplicate_forms(tmp_path: Path)
     prepare_domain_path(
         domain_dir,
         """
-        version: "3.0"
+        version: "2.0"
         forms:
           reservation_form:
             required_slots:
@@ -693,7 +703,7 @@ def test_migrate_domain_with_multiple_files_with_duplicate_forms(tmp_path: Path)
     prepare_domain_path(
         domain_dir,
         """
-        version: "3.0"
+        version: "2.0"
         forms:
           reservation_form:
             required_slots:
@@ -720,7 +730,7 @@ def test_migrate_domain_from_dir_with_other_sections(tmp_path: Path):
     prepare_domain_path(
         domain_dir,
         """
-        version: "3.0"
+        version: "2.0"
         entities:
         - outdoor
         slots:
@@ -734,7 +744,7 @@ def test_migrate_domain_from_dir_with_other_sections(tmp_path: Path):
     prepare_domain_path(
         domain_dir,
         """
-        version: "3.0"
+        version: "2.0"
         intents:
         - greet
         forms:
@@ -753,6 +763,10 @@ def test_migrate_domain_from_dir_with_other_sections(tmp_path: Path):
 
     for file in domain_dir.iterdir():
         migrated = rasa.shared.utils.io.read_yaml_file(file)
+
+        migrated_training_data_version = migrated.get("version")
+        assert migrated_training_data_version == '"3.0"'
+
         if file.name == domain_file_one:
             assert migrated.get("entities") == ["outdoor"]
         elif file.name == domain_file_two:
@@ -763,7 +777,7 @@ def test_migrate_domain_raises_exception_for_non_domain_file(tmp_path: Path):
     domain_file = prepare_domain_path(
         tmp_path,
         """
-        version: "3.0"
+        version: "2.0"
         nlu:
         - intent: greet
           examples: |
@@ -794,7 +808,7 @@ def test_migrate_domain_raises_for_non_domain_files(tmp_path: Path):
     prepare_domain_path(
         domain_dir,
         """
-        version: "3.0"
+        version: "2.0"
         nlu:
         - intent: greet
           examples: |
['rasa/core/migrate.py', 'tests/core/test_migrate.py']