NO-JIRA: use scrape class which is more succinct

This is suggested in [1]. "monitoring" has done it to their ServiceMonitors [2]
with [3].

[1]. https://rhobs-handbook.netlify.app/products/openshiftmonitoring/collecting_metrics.md/#configuring-prometheus-to-scrape-metrics

[2]. https://github.com/openshift/cluster-monitoring-operator/blob/c6bac82b735ac3a5ee67a3899d1a24bf95ae7277/assets/prometheus-operator/service-monitor.yaml#L20

[3]. openshift/cluster-monitoring-operator#2492

Author: hongkailiu

install/0000_90_cluster-version-operator_02_servicemonitor.yaml

@@ -15,10 +15,8 @@ spec:
     port: metrics
     scheme: https
     tlsConfig:
-      caFile: /etc/prometheus/configmaps/serving-certs-ca-bundle/service-ca.crt
       serverName: cluster-version-operator.openshift-cluster-version.svc
-      certFile: /etc/prometheus/secrets/metrics-client-certs/tls.crt
-      keyFile: /etc/prometheus/secrets/metrics-client-certs/tls.key
+  scrapeClass: tls-client-certificate-auth
   namespaceSelector:
     matchNames:
     - openshift-cluster-version