Fix label links in MR system notes

What does this MR do and why?

This changes label change system notes in MRs to link to the merge request list instead of the issue list.

Screenshots or screen recordings

How to set up and validate locally

1. Change labels on an MR
2. Check the link in the generated system note

Note: This only applies to newly generated system notes. Old system notes will still link to the issues list. This is because we don't want to bust the global Markdown cache for this minor bug.

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.

Related to #327001 (closed)