Default on :cluster_vulnerabilities feature flag

What does this MR do and why?

Default on :cluster_vulnerabilities feature flag

• change default from false to true

Changelog: changed

Screenshots or screen recordings

How to set up and validate locally

1. Ensure your GDK has a GitLab Ultimate license
2. Ensure your GDK is connected to the GitLab Runner
3. Follow these docs to set up the kubernetes agent
4. Navigate to a project
5. Link the agent to a project
6. Ensure the feature flag is removed by running Feature.remove(:cluster_vulnerabilities)
7. Navigate to the agent
8. View the vulnerabilities

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 #343917 (closed)