Add warn mode action option

What does this MR do and why?

Add warn mode action option

• disable warn mode action in enforce mode (undoing some of the work from Add an enforcement option for policy editor (!202806 - merged))
• allow for warn mode action to be deleted (undoing some of the work from Add an enforcement option for policy editor (!202806 - merged))
• allow for multiple warn mode actions to be added
• update tests

EE: true

References

Screenshots or screen recordings

Scenario	Screenshot
Warn mode
Enforce mode

How to set up and validate locally

1. Upload a GitLab Ultimate license
2. Enable the security_policy_approval_warn_mode feature flag (http://gdk.test:3000/rails/features)
3. Navigate to a project/group => Secure => Policies => New policy => Merge Request Approval Policy
4. Verify the policy starts in enforcement_type = warn
5. Verify the warn action can be added multiple times
6. Verify the warn action can be removed
7. Change the enforcement type and the yaml value is updated to enforce
8. Verify the user cannot add the warn action in enforce mode

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #575739 (closed)