Resolve "Rename the `Master` role to `Maintainer`" Backend

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5993

What does this MR do?

Rename permission name "Master" to "Maintainer", from backend developer's perspective.

This is a continuation of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19080

No need to worry about API because it does not use permission level names, but level number.

• EE check for ProjectCreationLevel|Masters and Developers + Masters and so on...

Are there points in the code the reviewer needs to double check?

Since the change is big, can I have multiple reviews?

Why was this MR needed?

See #42751 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests changed for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #42751 (closed)