Fix the namespaces of EE API entities

What does this MR do and why?

The MR moves all EE 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.

References

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

Screenshots or screen recordings

NA

How to set up and validate locally

NA

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #263403