Create merge request assignees for merge requests with non-empty assignee_id but without existing assignees

After #37694 (closed) is addressed, we need to migrate existing data just like gitlab-foss!26496 (merged)

Fortunately, the classes for this exist, we just need to optimize their behavior (probably narrow the scope of merge requests that need data to be migrated) and invoke them in post-migrate migrations

After this we can ignore/remove the assignee_id column: #30535