Explicitly remove index by name

What does this MR do?

This fixes a reported problem with a migration (see #33084 (closed)). When upgrading from 12.2.x, we seem to have cases where two indexes are present on releases.project_id:

Multiple indexes found on releases columns [:project_id]. Specify an index name from index_releases_on_project_id, releases_project_id_idx

The index index_releases_on_project_id is the one we were wanting to drop. This is now made explicit in the migration and should be backported to 12.2 upwards.

The original MR that added the migration is gitlab-foss!31487 (merged) and landed in %12.2.