Create merge request assignees for merge requests with non-empty assignee_id but without existing assignees
After https://gitlab.com/gitlab-org/gitlab/issues/37694 is addressed, we need to migrate existing data just like https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/26496 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: https://gitlab.com/gitlab-org/gitlab/issues/30535
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