Make MergeRequestMergeabilityCheckWorker idempotent

That's currently running on low urgency and just for gitlab project and the job is already idempotent (meaning we could just set idempotent! at it).

Out of 329,661 jobs started and processed, 2,956 were duplicates.

The timings there aren't great, but still acceptable for the besteffort:

Most time as expected is spent on Gitaly:

Source: https://dashboards.gitlab.net/d/sidekiq-queue-detail/sidekiq-queue-detail?orgId=1&from=now-7d&to=now&var-PROMETHEUS_DS=Global&var-environment=gprd&var-stage=main&var-queue=merge_request_mergeability_check

Considering the timings and the considerable throughput we'll see in near future, that's a potential SLI breaker.