Swap bigint columns on merge_requests stage one

What does this MR do and why?

This MR is for swapping columns assignee_id merge_user_id updated_by_id milestone_id source_project_id on merge_requests table. And then remove unused indexes and FKs in the second migration.

After the FK validation finished over the weekend around 6th Sep 2025, we are ready to merge this MR and get it run next weekend.

References

Issue - Swap MergeRequest ID columns (Step 2) (#507695)

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 one (!203979 - merged)
• Confirm Swap bigint columns on merge_requests stage one (!203979 - merged) is deployed to prod
• Skip the migrations 20251016010314 and 20251016010315 via ChatOps only for main
• Let the PDM job execute them for ci and sec
• Execute migrations for main manually via CR

Screenshots or screen recordings

NA

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.