[Audit Events] Synchronous database index addition

What does this MR do and why?

[Audit Events] Synchronous database index addition

This commit is step 2 to to add a migration to create the index_audit_events_on_entity_id_and_entity_type_and_created_at database index asynchronously on GitLab.com.

The asynchronous index was added in !120581 (merged)

related to #337757 (closed) resolves #411129 (closed)

How to set up and validate locally

1. bundle exec rails db:migrate
2. bundle exec rails db:rollback:main
3. bundle exec rails db:migrate

MR acceptance checklist

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