Add bulk vulnerability severity override audit events

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/180527



Merged-by: Alex Pooley <apooley@gitlab.com>
Approved-by: Miki Amos <mamos@gitlab.com>
Approved-by: Alex Pooley <apooley@gitlab.com>
Approved-by: Illya Klymov <iklymov@gitlab.com>
Reviewed-by: Miki Amos <mamos@gitlab.com>
Co-authored-by: Gal Katz <gkatz@gitlab.com>