[package] boost/all: Use `tools.cppstd_flag` from Conan 1.24
### Package and Environment Details (include every applicable attribute)
  * Package Name/Version: **boost/all**


The recipe is using undocumented Conan API:

https://github.com/conan-io/conan-center-index/blob/9419b6420fe94a453391ccd738bc70eb1c491179/recipes/boost/all/conanfile.py#L3

Conan 1.24 (to be released soon) provides new tool `tools.cppstd_flag` (https://github.com/conan-io/conan/pull/6744) that should replace it.
diff --git a/recipes/boost/all/conanfile.py b/recipes/boost/all/conanfile.py
index 70052f9f2..918c27d0c 100644
--- a/recipes/boost/all/conanfile.py
+++ b/recipes/boost/all/conanfile.py
@@ -1,7 +1,6 @@
 from conans import ConanFile
 from conans import tools
-from conans.client.build.cppstd_flags import cppstd_flag
-from conans.tools import Version
+from conans.tools import Version, cppstd_flag
 from conans.errors import ConanException
 
 from conans.errors import ConanInvalidConfiguration
@@ -568,12 +567,7 @@ class BoostConan(ConanFile):
         flags.append("toolset=%s" % self._toolset)
 
         if self.settings.get_safe("compiler.cppstd"):
-            flags.append("cxxflags=%s" % cppstd_flag(
-                    self.settings.get_safe("compiler"),
-                    self.settings.get_safe("compiler.version"),
-                    self.settings.get_safe("compiler.cppstd")
-                )
-            )
+            flags.append("cxxflags=%s" % cppstd_flag(self.settings))
 
         # CXX FLAGS
         cxx_flags = []
['recipes/boost/all/conanfile.py']