When using the issue header action to create a new issue,
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68226 added the
text "Related to #<parent-issue>." to the new issue's description.

This improves the UX of that change by:

- Renaming the "New issue" menu item to "New related issue" to clarify
  that it will create a link.
- Replacing the added description text with a new checkbox widget, to
  avoid obliterating the issue template. This checkbox widget is only
  shown when the new `add_related_issue` URL query parameter is present.

Changelog: changed