Retry post import

This MR adds import_failure retry mechanism introduced in #39100 (closed) for post import items as well.

It seems that @project.merge_requests.set_latest_merge_request_diff_ids for big projects can take a long time to execute, and we can retry this operation at least 3 times before failing, this will increase chances for this to succeed, without having big impact on application.

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #39107 (closed)