Make the FK on `Project#creator_id` VALID
Follow up from #390641 (closed)
We added the NOT VALID foreign key in !112126 (merged), and now we have to make it VALID.
Follow the database guides at https://docs.gitlab.com/ee/development/database/add_foreign_key_to_existing_column.html#schedule-the-fk-to-be-validated to get this done.