Backport of 'Add migrations for missing merge_requests stage 2 indexes for bigint'

What does this MR do and why?

Users are reporting issue raised during bigint migration, Upgrade from 18.7.1 to 18.8.0 fails in DB migra... (#586512 - closed)

The root cause would be, some existing indexes from merge_requests table are not present in DB (user DB derived from schema). When we swap bigint indexes, exception raised as those normal indexes do not exist.

In this MR, we add one more migration before the migration for swapping stage 2 bigint columns, so any missing indexes will be picked up. The timestamp in this MR, 20260102015627,is right before the swapping stage 2 bigint columns 20260102015629.

The initial MR has merged and deployed into master now.

References

• Issue - Upgrade from 18.7.1 to 18.8.0 fails in DB migra... (#586512 - closed)
• MR - Add migrations for missing merge_requests stage... (!220737 - merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.