Fix incorrect CVSSRating JSON field name

What does this MR do?

The JSON output for CVSSRating.Vector uses vector_string whereas the schema expects vector as a mandatory field. With this change, the JSON output now passes validation for v15.0.7 and newer schemas.

What are the relevant issue numbers?

n/a

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Ensure the report version matches the equivalent schema version
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer