In https://gitlab.com/gitlab-org/gitlab/merge_requests/23754, we added a
CTE optimization fence to ensure loading a user's projects is always
fast.

However, using a CTE for the dashboard/projects/starred endpoint is less
efficient than the default query because that endpoint also loads public
and internal projects. For now, we can just restrict use of this CTE for
the `index` action.

In the future, we may want to put this logic inside ProjectsFinder.

Closes https://gitlab.com/gitlab-org/gitlab/issues/201994