Part 2: Advanced Search - backfill milestone state in work items index

What does this MR do and why?

This MR adds a migration to backfill milestone state data in the work items search index, enabling filtering by upcoming and started milestones in Advanced Search (!197924 (merged)).

References

Part 1 MR

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Run Elastic::MigrationWorker.new.perform in rails c to execute the newly added migration
• Retrieve migration status with curl "http://localhost:9200/gitlab-development-migrations/_doc/20250718114546" | jq
  • In the output you should see that the migration successfully completed "completed": true
  • you can also check migration status by running bundle exec rake gitlab:elastic:info
  • sometimes Elastic::MigrationWorker is stuck, you can check that in http://localhost:3000/admin/sidekiq/scheduled. Restarting sidekiq should help gdk restart rails-background-jobs.

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.

Related to #547410 (closed)