Now that we've migrated to use
ProjectFeature#container_registry_access_level instead of
Project#container_registry_enabled, allow
container_registry_access_level to be modified by the public
projects API.

This allows users to set the visibility of a
container registry independently of the project visibility.

Also update the public API docs.

Deprecate container_registry_enabled in the public API in favor of
container_registry_access_level.

Changelog: deprecated