This benefits the projects API for unauthenticated calls.

Relevant query:

```sql
SELECT
    "projects".*
FROM
    "projects"
WHERE
    "projects"."visibility_level" = 20
    AND "projects"."pending_delete" = FALSE
ORDER BY
    "projects"."last_activity_at" ASC,
    "projects"."id" DESC
LIMIT 20 OFFSET 0
```

Plan without index: https://explain.depesz.com/s/t73N
Plan with index: https://explain.depesz.com/s/V45y

Very similar for the `last_activity_at DESC` sort order.

Relevant issue: https://gitlab.com/gitlab-org/gitlab/issues/36390

Related MR with more explanation:
https://gitlab.com/gitlab-org/gitlab/merge_requests/22623