Resolve: Reset resolved_at column of vulnerabilities table while unresolving the records

What does this MR do and why?

This migration fixes inconsistencies where vulnerabilities have a resolved_at timestamp but are not in the resolved state (state = 3).

References

• Reset `resolved_at` column of `vulnerabilities`... (#512857 - closed) • Charlie Kroon • 18.5
• Relevant discussion: #512857 (comment 2756749467)
• https://docs.gitlab.com/development/database/batched_background_migrations/

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