Dogfooding Merge Trains for runner

We should dogfood merge trains

documentation: https://docs.gitlab.com/ee/ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/

This feature is helpful for any development team

Candidates:

• gitlab-runner covered by #5070 (closed)
• ?
• gitlab-ce/gitlab-ee are already covered by https://gitlab.com/gitlab-org/gitlab-ce/issues/57190.
• www-gitlab-com covered by gitlab-com/www-gitlab-com#5177 (closed)