Merge branch 'minac_fix_vulnerability_notes_summary' into 'master'

Fix nondeterministic `discussions_for_summary` method

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/171131



Merged-by: Brian Williams <bwilliams@gitlab.com>
Approved-by: Brian Williams <bwilliams@gitlab.com>
Co-authored-by: Mehmet Emin INAC <minac@gitlab.com>