Enable Advanced SAST partial scan by default

Summary

At the time of writing, we are in the process of refining Faster Advanced SAST: Diff-based scanning in MRs (&16790 - closed). Due to the potential for false negatives, we've chosen to disable this feature by default for projects using Scan Execution Policies, to avoid users to enable it inadvertently without the explicit configuration of the policy owner.

Once we implement Incremental scanning for Advanced SAST (skip un... (&15545), a more robust form of partial scanning, we should enable this feature by default. When that happens, we'll also need to update the corresponding code that sets defaults this value.

Code reference from this comment by groupsecurity policies

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/security/security_orchestration_policies/scan_pipeline_service.rb#L21 is where we are setting default/restricted variables;
• then https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/security/security_orchestration_policies/ci_action/template.rb is where we are building the config for individual scan.