Update gl-sast-report.json to valid schema

What does this MR do?

This MR updates the start_time and end_time fields to valid values. As we are now validating schema for reports, this is currently causing any pipelines that use this schema to throw an error. See https://gitlab.com/gitlab-examples/security/security-reports/-/pipelines/546007859/security for an example.

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 (also in CE)
  • [-] Job definition example
  • [-] CI Templates (to be removed)
  • [-] Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• [-] Conforms to the Go guidelines
• Security reports checked/validated by reviewer