Initialize conversion of deployment_merge_requests to bigint

What does this MR do and why?

Initialize conversion of deployment_merge_requests to bigint

This MR creates the new bigint columns, installs triggers to keep them in sync for new records, and queues batched background migrations to backfill the existing records.

This is step one (release N) of migrating integer primary keys to bigint.

References

https://gitlab.com/gitlab-org/gitlab/-/issues/549017

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Related to #549017