Add override action to vulnerability management policy

What does this MR do and why?

Add override action to vulnerability management policy

Adds the override action type to the vulnerability management policy editor. This includes:

• Feature flag for severity customization
• Radio button UI for action type selection
• Override action component with customize dropdown
• Support for set_to, increase_by_one, decrease_by_one options
• Related tests for the action functionality

References

Screenshots or screen recordings

Scenario	Screenshot
Feature Flag - On
Feature Flag - Off

How to set up and validate locally

1. Upload a GitLab Ultimate license
2. Enable the security_policies_severity_customize feature flag at http://gdk.test:3000/rails/features
3. Navigate to a project/group => Secure => Policies => New policy => Vulnerability Management
4. Verify the user can select the Override action
5. Verify the action is updates with the new action and the yaml is updated

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 #588537