Add evidence to each vulnerability in the JSON report

What does this MR do?

Combines the alerts[].instance[].evidence and alerts[].otherinfo into a summary. The summary is exposed as vulnerabilities[].evidence.summary in the JSON output.

This MR in part resolves issue gitlab-org/gitlab#37027 (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
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer