Rename Vulnerabilities::OccurrenceSerializer to Vulnerabilities::FindingSerializer

Summary

Rename Vulnerabilities::OccurrenceSerializer to Vulnerabilities::FindingSerializer as described in #32963 (closed). This is to make the naming terminology consistent within the feature.

Improvements

The benefits of this refactoring are more clear (and correct) codebase in regards to the terminology which has already proven itself being complex enough to avoid any additional confusion.

Risks

Loss of functionality if a renaming is missed.

Involved components

• Vulnerabilities::OccurrenceSerializer (ee/app/serializers/vulnerabilities/occurrence_serializer.rb)
• VulnerabilitiesHelper (ee/app/helpers/vulnerabilities_helper.rb)
• ProjectVulnerabilityFindingsActions (ee/app/controllers/concerns/project_vulnerability_findings_actions.rb)