BE: Clean up the security policy pipeline execution action experiment

The security policy pipeline execution action experiment was introduced in %16.8. but we decided to go a different route with &13266 (closed). The new policy type will be released in %17.2. With this we can end the experiment and clean up all related code.

Implementation plan

• Remove the experiment from the docs.
• Remove the group setting.
• Remove related code from scan execution policy backend and ignore the toggle_security_policy_custom_ci and lock_toggle_security_policy_custom_ci columns on the NamespaceSetting model.
• Remove related definitions from security_orchestration_policy.json
• Create a follow-up issue to drop the columns in the next milestone.