Add optional CVSS vectors field to Vulnerability struct

What does this MR do?

This MR adds an optional cvss_vectors field to the Vulnerability struct so that it can be used in reports that support security report schema 15.0.7.

What are the relevant issue numbers?

Closes gitlab-org/gitlab#425958 (closed)

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