Remove extra index on snippets.project_id

Merged Markus Koller requested to merge fix-snippets-index-migration into master

What does this MR do?

This fixes a reported problem with a migration (see #37361 (closed)). When upgrading from previous versions, we seem to have cases where two indexes are present on snippets.project_id:

Multiple indexes found on snippets columns [:project_id]. Specify an index name from index_snippets_on_project_id, snippets_project_id_idx

The index index_snippets_on_project_id is the one we were wanting to drop. This is now made explicit in the migration and should be included in the next patch release for 12.5.

The original MR that added the migration is !18942 (closed) and landed in %12.5.