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)

app/models/protected_branch.rb

@@ -5,7 +5,7 @@ class ProtectedBranch < ActiveRecord::Base
   protected_ref_access_levels :merge, :push
 
   def self.protected_ref_accessible_to?(ref, user, project:, action:, protected_refs: nil)
-    # Masters, owners and admins are allowed to create the default branch
+    # Maintainers, owners and admins are allowed to create the default branch
     if default_branch_protected? && project.empty_repo?
       return true if user.admin? || project.team.max_member_access(user.id) > Gitlab::Access::DEVELOPER
     end