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

EE port at https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5948

Follow up (changes for backend): https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19413

What does this MR do?

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

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?

Internal method call/constant changes are in the followup MR, because:

• reduce probability of conflict
• separate the safe changes from risky changes, so we can rollback risky changes in case something goes wrong, but keep safe changes.

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
• 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)

doc/workflow/gitlab_flow.md

@@ -131,7 +131,7 @@ There is room for more feedback and after the assigned person feels comfortable
 If the assigned person does not feel comfortable they can close the merge request without merging.
 
 In GitLab it is common to protect the long-lived branches (e.g. the master branch) so that normal developers [can't modify these protected branches](http://docs.gitlab.com/ce/permissions/permissions.html).
-So if you want to merge it into a protected branch you assign it to someone with master authorizations.
+So if you want to merge it into a protected branch you assign it to someone with maintainer authorizations.
 
 ## Issue tracking with GitLab flow