Update common to v2.1.6

What does this MR do?

Bump common dependency to version: v2.1.5

The expectation diff is hard to read but it's really just an order change:

❯ cat test/expect/gl-sast-report.json | jq '.vulnerabilities | length'
33

❯ cat test/fixtures/gl-sast-report.json | jq '.vulnerabilities | length'
33

❯ diff <(cat test/expect/gl-sast-report.json | jq '.vulnerabilities | map(.cve) | sort') <(cat test/fixtures/gl-sast-report.json | jq -S '.vulnerabilities | map(.cve) | sort')

What are the relevant issue numbers?

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