Use a Keep to clean up duplicate indexes automatically

The gitlab-housekeeper can be used to automatically create MRs for routine maintenance. See the README.md to understand how to contribute.

We can add a new keep to https://gitlab.com/gitlab-org/gitlab/-/tree/master/keeps which looks at unused indexes and creates a migration that deletes it and opens an MR for it.

We can associate the index to the correct team using feature categories and assign a random backend engineer for review like we do in !139747 (diffs)