Finalize merge_request_diffs.project_id backfill

What does this MR do and why?

Finalize batched background migration for backfill of merge_request_diffs.project_id. The migration is complete on GitLab.com:

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #429174 (closed)