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

Fixed vale issues

See merge request gitlab-org/gitlab!60243

Fix Vale issues for ./integration/elasticsearch.md

See merge request gitlab-org/gitlab!60203

Fix Vale issues for unit_test_reports.md and ssh_certificates.md

See merge request gitlab-org/gitlab!60215

WTD-21 - Update fail_fast_testing.md

See merge request gitlab-org/gitlab!60216

Change of future tense to present

See merge request gitlab-org/gitlab!60225

fix vale future tense warnings

See merge request gitlab-org/gitlab!60240

Update polling.md by fixing present tense

See merge request gitlab-org/gitlab!60237

Fixed vale issues

See merge request gitlab-org/gitlab!60236

Addressing issues flagged by Vale

See merge request gitlab-org/gitlab!60233

Update vale issues doc/integration/saml.md

See merge request gitlab-org/gitlab!60234

Fix Vale issues for ./policy/maintenance.md

See merge request gitlab-org/gitlab!60238

Changed occurrences of future tense to present tense

See merge request gitlab-org/gitlab!60232

Fix tense issues raised by Vale

See merge request gitlab-org/gitlab!60226

Documentation: Fixes Vale issues with scim_setup.md

See merge request gitlab-org/gitlab!60227

Fix future tense in gitlab_com.md

See merge request gitlab-org/gitlab!60230

Fix Update gitea

See merge request gitlab-org/gitlab!60229

Fix Vale issues

See merge request gitlab-org/gitlab!60223

Update database_load_balancing.md

See merge request gitlab-org/gitlab!60220

Fix vale issues

See merge request gitlab-org/gitlab!60219

Update delete_account.md

See merge request gitlab-org/gitlab!60221

Part of Write the Docs 2021 Writing Day

See merge request gitlab-org/gitlab!60213

Fix future tense issues

See merge request gitlab-org/gitlab!60218