Backfill target_project_id for ComplianceViolation

What does this MR do and why?

Creates a batched background migration to backfill target_project_id if it is null for the merge_requests_compliance_violations table. It resorts to using the foreign key, merge_requests.target_project_id value

After this is merged there will be two follow-up additions:

• Add this to the ComplianceViolation model: belongs_to :target_project, class_name: 'Project', optional: false
• Add a new migration to make target_project_id NOT NULL so we can use it as a sharding key.

References

• #464719

MR acceptance checklist

• Plan on UPDATE: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/33620/commands/103444

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