Expose status metadata in VulnerabilityEntity

What does this MR do?

This MR exposes resolved_at, confirmed_at, and dismissed_at fields from Vulnerability model in VulnerabilityEntity. Related issue: #213834 (closed)

Does this MR meet the acceptance criteria?

Conformity

• This functionality is behind :first_class_vulnerabilities feature flag. This feature flag is off by default so no changelog entry is needed.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done