Merge branch 'jivl-add-top-margin-related-merge-requests' into 'master'

Add top margin to the related merge requests section

Closes #46697

See merge request gitlab-org/gitlab-ce!19379

app/views/projects/issues/_merge_requests.html.haml

 - if @merge_requests.any?
+  .prepend-top-default
     %h2.merge-requests-title
       = pluralize(@merge_requests.count, 'Related Merge Request')
     %ul.unstyled-list.related-merge-requests