Background migration to backfill merge request compliance violations

Description

Once we create the required columns in #385499 (closed), we'll have to backfill the existing rows in the merge request compliance violations table so that we can start using the data for queries.

Implementation Plan

1. Iterate over each row inside merge_request_compliance_violations table and update the newly created columns using the existing data in merge_requests and merge_request_metrics table.
2. This would be an irreversible background migration.