Change message to title in sarif.go

What does this MR do?

In the Updates to the 15-x-x schema with %16.0 deprecations, we did not update our SARIF conversion logic to output the title field in place of message. This is resulting in vulnerability titles like <code data-sourcepos="3:263-3:293">CWE-89 in SQLiteRoleProvider.cs</code> instead of the more descriptive message field "Sensitive cookie without 'HttpOnly' flag"

This MR updates sarif.go to output Message as Title

What are the relevant issue numbers?

gitlab-org/gitlab#414742 (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