When searching for merge requests in the context of a group we don't
need to use an Elasticsearch join. The join logic [defined in
ApplicationClassProxy](
https://gitlab.com/gitlab-org/gitlab/-/blob/339e55b484ce7f4250ef5bbb451c9ed2eac7c6e2/ee/lib/elastic/latest/application_class_proxy.rb#L145
) is not necessary as we are already filtering down based on the project
ID list the user has access to and hence we can just use the
`target_project_id` field instead of joining to the project and using
the ID field.

This MR is basically identical to
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46461 but in this
case we need to use `target_project_id` instead of `project_id` since
the MR only had `target_project_id` until recently and `project_id` is
not yet backfilled.

This change on it's own will make searches more performance (since this
term query is more efficient than a join) but it will additionally be
the next step in allowing us to move merge requests to a separate index
in Elasticsearch https://gitlab.com/groups/gitlab-org/-/epics/5468

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