Sync removal index_merge_requests_on_id_and_prepared_at

What does this MR do and why?

Create the sync migration for index_merge_requests_on_id_and_prepared_at index removal

Original MR: !127737 (merged)

Related issue: #419917 (closed)

Docs: https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#add-a-migration-to-destroy-the-index-synchronously

Current Indexes for merge_request table: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/22198/commands/71741