Remove security_on_demand_scans_feature_flag and release On-Demand Scans

we're using security_on_demand_scans_feature_flag on both the frontend and backend to selectively enable the on-demand dast scans feature. once we're satisfied that it's ready to roll out we should:

1. deploy to GitLab.com, disabled default, and selectively enable and observe.
2. enable by default, and observe. #237796 (closed)
3. when it looks good, remove feature flag.