Automatically migrate old merge request diffs to object storage - take two

What does this MR do?

This is like https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25432 , but attempts to avoid a difficult scheduler that scans the merge_request_diffs worker by enqueuing migration operations in response to actions taken by the user (close an MR, merge an MR, push to an MR).

This means we lose the "after 7 days" part of the spec, but also means we can provide most of the requested feature without too much difficulty.

TODO:

• On push
• On merge
• On close

@jramsay @DowueM @NikolayS WDYT to this alternative approach? The code is still quite shaky, and I've not tackled the "on push" case yet (I'm guessing we just need to touch MergeRequests::UpdateService), but it definitely means we drop the 7-day cutoff. There's been basically no movement on the other MR for a fortnight, and I'm concerned that it could miss %11.10 as-is.

What are the relevant issue numbers?

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
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #54670 (closed)