Rename Vulnerabilities::OccurrenceSerializer to FindingSerializer

What does this MR do?

This MR renames classes, files, and specs related to Vulnerabilities::OccurrenceSerializer to Vulnerabilities::FindingSerializer, as described in #214567 (closed).

This MR is part of a larger renaming effort as described in &3148 (closed).

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

Availability and Testing

The risk associated with this renaming is low, and the risk is a loss of functionality if a renaming is missed.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.