Change yaml format for pipeline execution policy

What does this MR do and why?

Change yaml format for pipeline execution policy

In order to have consistent format for both policy editor and policy.yml file, we need to adjust how yaml is rendered in a policy editor

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Description	UI
New format

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable ff

   Feature.enable(:security_policies_new_yaml_format)

   1. Go to Group/Project
   2. Secure -> Policies
   3. New pipeline execution policy
   4. Or edit existing
   5. Try to modify yaml and rule mode

Related to #497402 (closed)