Adding first registry in CLI erases the default one
### Describe the bug

By default, registry list in CLI contains a default registry `registry.quarkus.io` and there is an option to add new registries. Although, if user adds a new registry, e.g. `registry.quarkus.redhat.com`, then the default registry will be silently removed from the list and became inaccessible.

### Expected behavior

Default registry stays in the list, registries are never removed without explicit user request.

### Actual behavior

Default registry is removed from the list.

### How to Reproduce?

1. Install quarkus afresh or remove/rename file ~/.quarkus/config.yaml
2. Check default registry.
```
$ quarkus registry list
Configured Quarkus extension registries:
registry.quarkus.io
```
3. Check existing extensions. 
```
$ quarkus ext list | wc -l
920
```
4. Add non-default registry: ` quarkus registry add registry.quarkus.redhat.com`
5. Check registry list:
```
$ quarkus registry list
Configured Quarkus extension registries:
registry.quarkus.redhat.com #registry.quarkus.io is not in the list
(Read from ~/.quarkus/config.yaml)
```
6. Check existing extensions. 
```
$ quarkus ext list | wc -l
170 # at least some extensions are not accessible any more
```

### Output of `uname -a` or `ver`

5.14.17-201.fc34.x86_64

### Output of `java -version`

Java version: 11.0.13, vendor: Eclipse Adoptium

### GraalVM version (if different from Java)

_No response_

### Quarkus version or git rev

2.5.1.Final

### Build tool (ie. output of `mvnw --version` or `gradlew --version`)

Apache Maven 3.8.4

### Additional information

_No response_
diff --git a/devtools/cli/src/main/java/io/quarkus/cli/RegistryAddCommand.java b/devtools/cli/src/main/java/io/quarkus/cli/RegistryAddCommand.java
index f9f2c5fc46a..c9d7449d4b9 100644
--- a/devtools/cli/src/main/java/io/quarkus/cli/RegistryAddCommand.java
+++ b/devtools/cli/src/main/java/io/quarkus/cli/RegistryAddCommand.java
@@ -1,5 +1,7 @@
 package io.quarkus.cli;
 
+import static io.quarkus.registry.Constants.DEFAULT_REGISTRY_ID;
+
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
@@ -48,7 +50,16 @@ public Integer call() throws Exception {
         if (persist) {
             output.printText("Configured registries:");
             for (RegistryConfig rc : config.getRegistries()) {
-                output.printText("- " + rc.getId());
+                if (!existingConfig && config.getRegistries().size() == 1
+                        && !rc.getId().equals(DEFAULT_REGISTRY_ID)) {
+                    output.warn(
+                            rc.getId() + " is the only registry configured in the config file.\\n" + rc.getId()
+                                    + " replaced the Default registry: "
+                                    + DEFAULT_REGISTRY_ID);
+                } else {
+                    output.printText("- " + rc.getId());
+                }
+
             }
             if (configYaml != null) {
                 config.persist(configYaml);
['devtools/cli/src/main/java/io/quarkus/cli/RegistryAddCommand.java']