As part of our efforts to avoid using joins in Elasticsearch queries
https://gitlab.com/groups/gitlab-org/-/epics/2054 we need to
"denormalize" (copy them into child docs) the permission related fields
needed for searching.

In order to allow us to search for merge requests without joining to the
project we need to store the `merge_requests_access_level` as well as
the `visibility_level` of the project on the merge request record. In
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/59731 we started
storing these fields for any newly created or updated Merge Requests.

This MR is to backfill all the existing MRs in the index that don't have
this value set. Technically we are backfilling 3 fields but, since all
these fields were added in that MR, we can just check for the existence
of any of those fields to be sure that a record has them all.

This MR does not yet use these fields as it's only the 2nd step of all
the steps described in https://gitlab.com/groups/gitlab-org/-/epics/5468

Once these fields are backfilled we can start using them and get some
performance gains. Technically we could add that code in this MR to use
these new fields (conditioned upon the migration being finished) but in
the past we've found that we often ran into issues that were only with
one part of the code that led to us needing to revert and then do the
whole process again and code review again and this is less efficient.

This MR is actually pretty much an identical copy of the same thing we
already did for issues a while back in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/47819 but all I
did was find and replace `issues` with `merge_requests` since that's the
only difference.

Changelog: changed