Deprecate legacy feature flags api

Add new version feature flags api

doc/api/feature_flag_specs.md

@@ -2,7 +2,10 @@
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/9566) in [GitLab Premium](https://about.gitlab.com/pricing/) 12.5.
 
-The API for creating, updating, reading and deleting [Feature Flag Specs](../user/project/operations/feature_flags.md#define-environment-specs).
+CAUTION: **Deprecation**
+This API is deprecated and [scheduled for removal in GitLab 14.0](https://gitlab.com/gitlab-org/gitlab/-/issues/213369).
+
+The API for creating, updating, reading and deleting Feature Flag Specs.
 Automation engineers benefit from this API by being able to modify Feature Flag Specs without accessing user interface.
 To manage the [Feature Flag](../user/project/operations/feature_flags.md) resources via public API, please refer to the [Feature Flags API](feature_flags.md) document.
 

doc/user/project/operations/feature_flags.md

@@ -311,6 +311,6 @@ end
 You can create, update, read, and delete Feature Flags via API
 to control them in an automated flow:
 
-- [Feature Flags API](../../../api/feature_flags.md)
+- [Legacy Feature Flags API](../../../api/feature_flags_legacy.md)
 - [Feature Flag Specs API](../../../api/feature_flag_specs.md)
 - [Feature Flag User Lists API](../../../api/feature_flag_user_lists.md)