Implement resolved/no longer detected findings alert on Security MR widget

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Why are we doing this work

We want to give visibility to and confirmation of any vulnerabilities merge request authors have successfully fixed with their changes, which we don't currently surface very clearly.

Relevant links

Information that the developer might need to refer to when implementing the issue.

• Design Issue
• Issue this was extracted from
  • Discussion where it was decided to delay implementing this until either https://gitlab.com/groups/gitlab-org/-/epics/4690 or #12896[c1-sec-tab_vulns-detected.png] is further along

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

frontend

• Add getter for sum of all "fixed" security findings to Vuex
• Use slot added to ReportSection as part of #267504 (closed) to render success alert if there are any fixed security findings