Merge branch '238580-dast-site-validation-rollout-feature-flag' into 'master'

Create security_on_demand_scans_site_validation feature flag

Closes #238580

See merge request gitlab-org/gitlab!40685

config/feature_flags/development/security-on-demand-scans-site-validation.yml

+---
+name: security-on-demand-scans-site-validation
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40685
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/241815
+group: group::dynamic analysis
+type: development
+default_enabled: false