WIP: Fix rendering of system notes on commits in merge requests

What does this MR do?

When individual notes on commits are shown in MRs, they are grouped into one discussion.

This MR excludes system notes from this grouping.

What are the relevant issue numbers?

Closes #55268 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer