Add feature flag for undetected_since in vulnerabilities over time chart

What does this MR do and why?

Add feature flag for undetected_since in vulnerabilities over time chart

This uses a new feature flag specially for guarding whether undetected_since is taken into account for the vulnerabilities over time chart on the new security dashboard.

References

Related #587193 (closed)

Screenshots or screen recordings

Feature flag disabled	Feature flag enabled

How to set up and validate locally

1. Make sure you've updated master such that !218189 (merged) changes are included
2. Enable http://gdk.test:3000/rails/features/new_security_dashboard_exclude_no_longer_detected
3. In a security reports example project, remove some vulnerabilities from sample json files (which should trigger a new pipeline run)
4. After pipeline run, verify in vulnerability report with "Activity: No longer detected" you actually have no longer detected vulnerabilities
5. Go to security dashboard and verify you don't see a drop in the vulnerabilities over time chart in regards to these no longer detected vulnerabilities
6. Enable new feature flag: http://gdk.test:3000/rails/features/new_security_dashboard_over_time_chart_undetected_since
7. Verify in security dashboard you can see a drop in respect to the no longer detected vulnerabilities

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.