This is the 2nd last step in our efforts to avoid using joins in
Elasticsearch and move merge requests to a new index
https://gitlab.com/groups/gitlab-org/-/epics/5468 . This changes the
behaviour of searches at the global (any group, any project) level to no
longer use joins (`has_parent`) queries and instead just use the new
`project_id` and `merge_requests_access_level` fields that were added to
the `merge_requests` type (denormalized).

These new fields needed to avoid joins are migrated in the migration
added in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/59836 .

This MR just needs to check that the migration is finished and if it is
we can set `no_join_project` which was implemented in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/48583 .

We also need to make sure we have tests for the state where the
migration is not run so we don't accidentally break permission logic for
unmigrated indices in any future refactorings.

Changelog: changed