[CI] handling v9 setting in benchmark marathon

[kaisecheng]

benchmark script failed to start in v8 due to unrecognised v9 setting xpack.monitoring.allow_legacy_collection
This commit removed the setting when the running version is less than 9

[mergify]

This pull request does not have a backport label. Could you fix it @kaisecheng? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[kaisecheng]

Cancelled the CI
It has passed the previous failure

[mashhurs]

🎉 Thank you!

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: bdbe14f