codeintel: Fix SelectRepositoriesForIndexScan for Cloud scale

Created by: efritz

The current implementation of SelectRepositoriesForIndexScan simply iterates all non-deleted repositories. We don't have infinite compute, so we need to cull the list of repositories we consider auto-indexing (at least on Cloud) before we make use of the method introduced in https://github.com/sourcegraph/sourcegraph/pull/25991.