Support model 15 of the schemas

What does this MR do?

Change fields of the Vulnerability struct type so that analyzers can generate reports that validate version 15-0-0 of the Security Report Schemas.

• Change Vulnerability.Scanner to a pointer, and set omitempty.
• Set omitempty for Vulnerability.Category.
• Set omitempty for Vulnerability.CompareKey.
• Set omitempty for Ref.CompareKey.
• Document Category, Message, and Confidence as deprecated.
• Release new minor version of report. This is a transition version, and the major version doesn't change.

What are the relevant issue numbers?

gitlab-org/gitlab#368140 (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