Add confirm & resolve GraphQL mutations for security findings

What does this MR do and why?

Add securityFindingConfirm and securityFindingResolve mutations, letting a security finding be set to Confirmed or Resolved from the pipeline Security tab/merge request page. We've only supported dismissing and reverting to detected of findings, but this brings parity with vulnerabilities.

Frontend implementation will come in later MR, with feature flag, for multi-version compatibility.

There's an explicit ask for these in #431818 and they've been planned for a while.

References

Related #431818

Screenshots or screen recordings

With FE POC applied locally, you can see the mutations in action for confirm and resolve:

How to set up and validate locally

1. Apply FE patch: fe-finding-new-mutations.patch
2. Import https://gitlab.com/gitlab-examples/security/security-reports and run pipeline on master
3. Go to the pipeline > Security tab. Select a finding with the checkbox. Select Confirmed or Resolved in Select status dropdown. Add comment. Click Change status.
4. Go to vulnerability report, find the vulnerability of the finding you just updated status and validate it has the status you just set.

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.