Update Knative Prow to use the new monitoring stack

[chizhg]

As per the Prow owners, Knative Prow is currently using a legacy monitoring stack (in https://github.com/knative/test-infra/tree/5094d5004d5b9188e508024789d0e0dee2b1a866/prow/cluster/monitoring), which has been deprecated in favor of https://github.com/GoogleCloudPlatform/oss-test-infra/tree/master/prow/oss/terraform.

Since the legcy monitoring stack is not actively being maintained, and the new one is easier to setup and measure more valid metrics, Prow team's recommendation is to make the switch to it.

Please reach out to Prow team via the k8s/Prow slack channel if there are any questions.

/cc @kvmware @upodroid

[github-actions]

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