Only create unmergeable todos once

What does this MR do?

Ensure that todos for unmergeable MRs are only created once per 'unmergeable event', even if the todo is marked as done.

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

Don't think so!

Why was this MR needed?

MWPS status persists even if the MWPS failed, and an MR can 'become unmergeable' multiple times as we update the target branch.

Does this MR meet the acceptance criteria?

• Changelog entry 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 https://gitlab.com/gitlab-org/gitlab-ce/issues/28555.