[Keyvault] az keyvault create: Add warning for upcoming breaking ch…

…ange that `--enable-rbac-authorization` will default to `true` (Azure#28178)

src/azure-cli/azure/cli/command_modules/keyvault/custom.py

@@ -671,6 +671,8 @@ def create_vault(cmd, client,  # pylint: disable=too-many-locals, too-many-state
         # if client.get raise exception, we can take it as no existing vault found
         # just continue the normal creation process
         pass
+    logger.warning('We will enable rbac authorization by default in the near future,'
+                   ' please manually specify --enable-rbac-authorization if you want to overwrite the default value.')
     from azure.cli.core._profile import Profile, _TENANT_ID
     from azure.cli.command_modules.role import graph_client_factory, GraphError