Reference default dir for DS_REPORT_URL

What does this MR do?

This MR updates the DS_REPORT_URL to point to the default subdir, since the location of the expectation has changed. See this discussion for more details.

This should be merged right before gitlab-org/security-products/analyzers/gemnasium-maven!189 (merged)

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
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer