Swap bigint columns on merge_requests stage two

What does this MR do and why?

This is the final MR where we swap stage 2 bigint columns for merge_requests table.

In before MRs, we have prepared indexes (async and sync), and FKs (async validation). And this MR, we perform below things,

• Add sync FK validation for non-GitLab.com envs
• Add final step for partition FK validation
• Swap stage 2 bigint columns (target_project_id, latest_merge_request_diff_id, last_edited_by_id) and related indexes and FKs
• Drop temp indexes, FKs after swap (leave columns for final removal together after one milestone stop)

References

• Issue - Swap MergeRequest ID columns (Step 2) (#507695)
• MR - Prepare bigint indexes for merge_requests stage... (!204153 - merged)
• MR - Prepare foreign keys for bigint conversion on m... (!212201 - merged)

Process Plan

This MR would follow unstandard process, and would not run on GitLab.com via normal PDM.

• Merge Swap bigint columns on merge_requests stage two (!216433)
• Confirm Swap bigint columns on merge_requests stage two (!216433) is deployed to prod
• Skip the migrations 20251215020900 and 20251215020901 via ChatOps only for main
• Let the PDM job execute them for ci and sec
• Execute migrations for main manually via CR

MR acceptance checklist

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