Update issue closing pattern to be more forgiving

What does this MR do?

1. Allows oxford commas (comma before and) when closing multiple issues in an MR (#17500 (closed))
2. Allows spaces before commas when closing multiple issues in an MR - I thought it would be a good idea to make it a bit more forgiving (without allowing false positives naturally)

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

Well, this is my first MR to Gitlab, so... 😅

I tried to extract the spaces-before-comma part out, so %{issue_ref} *(?:(?:,? +and +|, *)?) instead of %{issue_ref}(?:(?: *,? +and +| *, *)?), but it seems to confuse the regex engine. Compare on Rubular: this works, this doesn't.

Why was this MR needed?

To make closing issue extraction align more with what humans expect.

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
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #17500 (closed)