Updated infra monitoring Grafana dashboard thresholds

[judyer28]

Overview/Summary

Updated thresholds used for Grafana Infra Monitor dashboard. Added 4 thresholds for disks and updated App GW unhealthy host count.

This PR fixes/adds/changes/removes

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Breaking Changes

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As part of this Pull Request I have

• Read the Contribution Guide and ensured this PR is compliant with the guide
• Checked for duplicate Pull Requests
• Associated it with relevant GitHub Issues or ADO Work Items (Internal Only)
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Ensured PR tests are passing
• Updated relevant and associated documentation (e.g. Contribution Guide, Docs etc.)