Fix the namespaces of EE API merge train and CI entities

The MR moves all EE merge train and CI related entities that are in ee/lib/ee/api/entities/ to ee/lib/api/entities, and updates their namespace accordingly.

The fix is required to conform to the "Separation of EE code in the backend" rule for EE-only features. https://docs.gitlab.com/development/ee_features/#ee-only-features.