Manage dismissed vulnerabilities in the Group Security Dashboard

Problem to solve

The Group Security Dashboard shows dismissed vulnerabilities in the list, and they affect counters as well.

Users should be able to consider dismissed vulnerabilities as non-existing.

Proposal

Allow people to hide dismissed vulnerabilities and to remove them from counters in the Group Security Dashboard.

A possible option is to use a filter dropdown like we are doing in https://gitlab.com/gitlab-org/gitlab-ee/issues/6240, with Active, All, Dismissed.