Consider adding labels in the "Note on MergeRequest" webhook payload

This is a shame, but currently only issue notes have the noteable labels present in the payload: https://gitlab.com/gitlab-org/gitlab/-/blob/d9b30205dd90e5e24f27cb853c9365bba7d1701d/lib/gitlab/data_builder/note.rb#L47.

It was added in 2019 for the "Note on issue" webhook payload, not sure why this wasn't added for MRs at this time: ed8a00b9