[importlib] Update deprecate message for files function (#14773)

Author: donbarbos

stdlib/importlib/resources/_common.pyi

@@ -21,7 +21,7 @@ if sys.version_info >= (3, 11):
         @overload
         def files(anchor: Anchor | None = None) -> Traversable: ...
         @overload
-        @deprecated("First parameter to files is renamed to 'anchor'")
+        @deprecated("Deprecated since Python 3.12; will be removed in Python 3.15. Use `anchor` parameter instead.")
         def files(package: Anchor | None = None) -> Traversable: ...
 
     else: