Backfill milestone title and id in work items index

What does this MR do and why?

There is a part one MR where we introduced mappings for milestone_id and milestone_data in work_items index. We also made sure to trigger index update every time milestone title is editted.

In this MR we are going to backfill milestone_id and milestone_data for all existing docs in work_items index.

References

Related issue #525883 (closed)

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/20250425124136" | jq
  • In the output you should see that the migration successfully completed "completed": true

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 #525883 (closed)