Enable invalid_scan_result_policy_prevents_merge by default

What does this MR do and why?

This MR enables invalid_scan_result_policy_prevents_merge feature flag by default. The feature flag has been rolled out globally throughout 16.0 and there has been no concerning feedback.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #405023 (closed)