Reject system notes when indexing notes on work items

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175632



Merged-by: Dmitry Gruzd <dgruzd@gitlab.com>
Approved-by: Terri Chu <tchu@gitlab.com>
Approved-by: Dmitry Gruzd <dgruzd@gitlab.com>
Reviewed-by: Terri Chu <tchu@gitlab.com>
Co-authored-by: Joseph Snyder <joe.snyder@kitware.com>

(cherry picked from commit e9d27ce7)

c102c108 Ignore system notes when indexing notes on work items

Co-authored-by: Dmitry Gruzd <dgruzd@gitlab.com>