Update default scan execution yaml

What does this MR do and why?

Update default scan execution yaml

• it now specified required fields name and branches

Changelog: changed

EE: true

Screenshots or screen recordings

How to set up and validate locally

1. Ensure your GDK has a GitLab Ultimate license
2. Turn on the :scan_result_policy feature flag
3. Follow the snippet for creating scan execution policies
4. Navigate to Security & Compliance => Policies

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 #339751 (closed)