Merge branch 'ff-purge-stale-findings' into 'master'

FF rollout issue for purge_stale_security_findings

See merge request gitlab-org/gitlab!83346

config/feature_flags/ops/purge_stale_security_findings.yml

 ---
 name: purge_stale_security_findings
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/81423
-rollout_issue_url:
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/356464
 milestone: '14.9'
 type: ops
 group: group::threat insights