Disallow reopening of locked merge request

What does this MR do?

It prevents reopening of locked merge requests. This MR is basically a clone of !21299 (merged) for merge requests.

Note: I did not add specific tests as !21299 (merged) did, because app/policies/merge_request_policy is generally not tested at the moment (as opposed to issues_policy). I can add this if necessary.

What are the relevant issue numbers?

#56864 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Security reports checked/validated by reviewer

changelogs/unreleased/56864-reopen-locked-mr.yml

 ---
 title: Disallow reopening of a locked merge request
-merge_request:
+merge_request: 24882
 author: Jan Beckmann
 type: fixed