Update merge request diff files_count cache when removing diffs

What does this MR do?

The files_count cache column was created with the assumption that the number of files never changes throughout the life of a diff. However, we have discovered the DeleteDiffFilesWorker, which removes them (and sets the state of the diff to :without_files) in some limited cases.

This MR updates the cache columns (both commits_count and files_count) so they will be re-recorded on every update. Since updates are rare, the additional cost of this should be very low.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #240921 (closed)