Merge branch 'minac_474280_set_total_number_of_vulnerabilities_for_existing_projects' into 'master' 

Set `vulnerability_count` of project_security_statistics records

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167812



Merged-by: Gregory Havenga <11164960-ghavenga@users.noreply.gitlab.com>
Approved-by: Buck O'Leary <bucoleary@gitlab.com>
Approved-by: Alex Pooley <apooley@gitlab.com>
Approved-by: Nicolas Dular <ndular@gitlab.com>
Approved-by: Gregory Havenga <11164960-ghavenga@users.noreply.gitlab.com>
Reviewed-by: Nicolas Dular <ndular@gitlab.com>
Co-authored-by: Mehmet Emin INAC <minac@gitlab.com>