Security Dashboard improvements
### Description
Improvements after Security Dashboard MVC: https://gitlab.com/groups/gitlab-org/-/epics/161, based on the product discovery in https://gitlab.com/gitlab-org/gitlab-ee/issues/7091.
The Security Dashboard could be split into two different panels:
1. the Security Overview, where metrics are presented (target: Director of Security)
1. the list of vulnerabilities, where specific actions can be taken (target: Security Specialist)
epic