Add default value for policy scope for security policies

What does this MR do and why?

Policy scope field should present in default policy state for a policy on a group level.

It has to be in default policy both for scan execution and result policy.

Screenshots or screen recordings

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UI	Description
	Scan Execution Policy
	Scan Result Policy

How to set up and validate locally

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

Feature.enable(:security_policies_policy_scope)

1. Go to a group
2. Secure -> Policies -> New policy
3. Check both result and execution policy
4. Make sure policy_scope is in default rule in yaml format.
5. Go to a project check same flow, policy_scope should not present in default policy

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 #428494 (closed)