index_issues_on_namespace_id_iid_unique synchronous database index addition

Summary

This issue is to add a migration(s) to createthe index_issues_on_namespace_id_iid_unique database index(es) synchronously after it has been createdon GitLab.com.

At the same time, we are dropping the index_issues_on_namespace_id as the new one would be a duplicate of that one

The asynchronous index was introduced in !139827 (merged)