Prepopulate new issue with "/relate" link to parent

What does this MR do and why?

!68226 (merged) added the text Related to #<parent-issue>.. Now that GitLab supports the /relate quick action when creating an issue (#295493 (closed)), change that prepopulated text to /relate #<parent-issue>.

Screenshots or screen recordings

How to set up and validate locally

1. View an issue
2. At the top of the issue, click the issue header actions dropdown menu
3. Click "New issue"
4. Notice that the new issue form now uses the /relate quick-action (instead of having a plain-text link)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #198494 (closed) and #340529