Add merge_request_diffs.project_id index and foreign key

What does this MR do and why?

• Synchronously creates the index that was scheduled in !136800 (merged)
• Adds a NOT VALID foreign key on merge_request_diffs.project_id

I've confirmed this index has been created in both Main and CI using postgres.ai following https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#verify-the-mr-was-deployed-and-the-index-exists-in-production

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)