WIP: Completely disable the rebase merge method when unlicensed

What does this MR do?

Completely removes the rebase_merge strategy when unlicensed.

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

Why was this MR needed?

!2200 (merged) was an incomplete solution to #2570 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2570 (closed)

added 19 commits

• 4d69ca48...c67c4fb1 - 18 commits from branch gitlab-org:master
• e1ae630b - Completely disable the rebase merge method when unlicensed

Compare with previous version

marked as a Work In Progress

changed the description

Actually, I'm going to address this as part of !2272 (merged)

closed