Resolve "Notes API for issues no longer returns label additions/removals"

What does this MR do?

Fix a problem with the API that redacted some labels addition/removals for no obvious reason.

The problem was caused by the Commit.reference_pattern being too aggressive, and matching on seven consecutive digits. On Gitlab.com, it seems we have > 10e6 labels, so that would cause labels message to match as a cross-reference.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This is a regression.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #39461 (closed)