Maintain "force_remove_source_branch" options on Merge Request unless specified

What does this MR do?

Fixes a problem where Merge Requests were losing the state associated with the flag to remove the source branch upon merge

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

Are further tests required?

Why was this MR needed?

Several issues about this have been reported

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• 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 #23234 (closed)
• Closes #23191 (closed)
• Closes #19351 (closed)

Added 3 commits:

• 5a021044...d3a98380 - 2 commits from branch gitlab-org:master
• 1c2390b4 - Maintain "force_remove_source_branch" options on Merge Request unless specified

Thanks for taking care of this

Won't it close #22507 (closed) too?

Added 1 commit:

• 6f750232 - Maintain "force_remove_source_branch" options on Merge Request unless specified

Resolved all discussions

@eservais I'm still not convinced that this will fix Ben's issue. If it does then that's great, but I'd rather keep that issue around in case there is another problem there.

Reassigned to @rymai

Thanks a lot @markglenfletcher, nice find for an annoying bug (I'm setting it as ~"Pick into Stable" for %8.12 )! :)

Added ~149423 label

Milestone changed to %8.12

Reassigned to @markglenfletcher

Nice find @markglenfletcher. I used to think I forgot to check the box, but now I know there was some quiet problem hiding all along.

Added 1 commit:

• 467d4ef4 - Maintain "force_remove_source_branch" options on Merge Request unless specified

Resolved all discussions

Reassigned to @rymai

Enabled an automatic merge when the build for 467d4ef4 succeeds

Thanks @markglenfletcher!

That build! (╯°□°)╯︵ ┻━┻

@markglenfletcher Could you rebase? Maybe this is fixed on master already?