Check feature flag when deciding occurrence state

What does this MR do?

In support of First-class vulnerabilities backend MVC implementation, the state method was added to the Vulnerabilities::Occurrence model. 4c381288

This MR adds a check of :first_class_vulnerabilities feature flag to prevent unnecessary attempts to load vulnerabilities to determine the state of the occurrence.

There is already an outstanding issue to address the performance when the feature flag is set to true: #34003 (closed)

Screenshots

Screenshot of unnecessary Vulnerability Load calls that this MR avoids when the :first_class_vulnerabilities feature flag is off.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content