Clean-up phase for MR metrics population background migration

What does this MR do?

Finishes any remaining PopulateMergeRequestMetricsWithEventsData background migration. Additionally, stops special-casing the usage of events table data to serialize metrics information on the MR serializer.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #41587 (closed)