Use bulk-dismissal mutation on vulnerability report

What does this MR do and why?

It uses a new, dedicated multi-dismissal GraphQL mutation for bulk-dismissing vulnerability findings.

Note: This is currently behind the dismiss_multiple_vulnerabilities feature flag.

Screenshots or screen recordings

No visible changes.

How to set up and validate locally

1. Check out this branch
2. Run echo "Feature.enable(:dismiss_multiple_vulnerabilities)" | rails c
3. Navigate to a project with vulnerabilities, then go to Secure -> Vulnerability Report
4. Make sure that bulk-changes to vulnerabilities are applied correctly

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.