Deprecate Legacy Feature Flags

We are currently implementing new feature flags with a new data model.

We need to deprecate and then remove the legacy feature flags at some point.

This is a place holder issue to start thinking about a strategy and time frame.

API endpoints that will be deprecated:

• All of the Feature Flag Scopes API will be deprecated: https://docs.gitlab.com/ee/api/feature_flag_specs.html
• Parts of the Feature Flags API will be deprecated: https://docs.gitlab.com/ee/api/feature_flags.html

All the CRUD operations related to the existing feature flag scopes will be deprecated. This includes, for instance, the following parameters for the New feature flag endpoint:

• scopes
• scopes:environment_scope
• scopes:active
• scopes:strategies

Similarly, the scopes field on getting a feature flag will be deprecated.