Add the Issues /related_merge_requests endpoint

[fvarose]

This adds a missing endpoint for issues: /related_merge_requests.

I confirmed this works by testing with a local GitLab instance running via gdk:

pry(main)> Gitlab.related_merge_requests(7, 8).length
=> 2

pry(main)> Gitlab.related_merge_requests(7, 8)
=> [#<Gitlab::ObjectifiedHash:1560 {hash: {"id"=>28, "iid"=>1, "project_id"=>7, "title"=>"Dolorem id velit magnam ut ut aut beatae.", "description"=>"Molestiae quam corrupti sed facere id.", "state"=>"merged", ... }}]

[mnuzzo24]

Hi @NARKOZ

Would you be able to take a look at this, please?

Thanks in advance 🙏

cc : @fvarose

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[NARKOZ]

Added in #683
Thank you.