Bug 1868304

Description of problem:
we have added ValidatingWebhookConfiguration prometheusrules.openshift.io since 4.6, but it blocks monitoring downgrade from 4.6 to 4.5.
the workaroud is 
# oc delete ValidatingWebhookConfiguration prometheusrules.openshift.io
the downgrade would continue.

down grade from 4.6.0-0.nightly-2020-08-12-003456 to 4.5.0-0.nightly-2020-08-08-162221
# oc get co/monitoring -oyaml
...
  - lastTransitionTime: "2020-08-12T06:40:10Z"
    message: 'Failed to rollout the stack. Error: running task Updating Prometheus-k8s failed: reconciling Prometheus rules PrometheusRule failed: updating PrometheusRule object failed: Internal error occurred: failed calling webhook "prometheusrules.openshift.io": Post https://prometheus-operator.openshift-monitoring.svc:8080/admission-prometheusrules/validate?timeout=5s: no service port 8080 found for service "prometheus-operator"'
    reason: UpdatingPrometheusK8SFailed
    status: "True"
    type: Degraded
...
Note: in a healthy 4.6 cluster
# oc -n openshift-monitoring get svc prometheus-operator
NAME                  TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)             AGE
prometheus-operator   ClusterIP   None         <none>        8443/TCP,8080/TCP   6h23m

when it downgrades to 4.5, it only has 8443 port, which is expected for 4.5
# oc -n openshift-monitoring get svc prometheus-operator
NAME                  TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)    AGE
prometheus-operator   ClusterIP   None         <none>        8443/TCP   5h5m

# oc -n openshift-monitoring get svc prometheus-operator -oyaml
apiVersion: v1
kind: Service
metadata:
  annotations:
    service.beta.openshift.io/serving-cert-secret-name: prometheus-operator-tls
  creationTimestamp: "2020-08-12T03:49:15Z"
  labels:
    app.kubernetes.io/component: controller
    app.kubernetes.io/name: prometheus-operator
    app.kubernetes.io/version: v0.38.1
  managedFields:
  - apiVersion: v1
    fieldsType: FieldsV1
    fieldsV1:
      f:metadata:
        f:annotations:
          .: {}
          f:service.beta.openshift.io/serving-cert-secret-name: {}
        f:labels:
          .: {}
          f:app.kubernetes.io/component: {}
          f:app.kubernetes.io/name: {}
          f:app.kubernetes.io/version: {}
      f:spec:
        f:clusterIP: {}
        f:ports:
          .: {}
          k:{"port":8443,"protocol":"TCP"}:
            .: {}
            f:name: {}
            f:port: {}
            f:protocol: {}
            f:targetPort: {}
        f:selector:
          .: {}
          f:app.kubernetes.io/component: {}
          f:app.kubernetes.io/name: {}
        f:sessionAffinity: {}
        f:type: {}
    manager: operator
    operation: Update
    time: "2020-08-12T06:36:52Z"
  name: prometheus-operator
  namespace: openshift-monitoring
  resourceVersion: "163229"
  selfLink: /api/v1/namespaces/openshift-monitoring/services/prometheus-operator
  uid: 56e0efe0-81e2-48e5-b75a-3df951163071
spec:
  clusterIP: None
  ports:
  - name: https
    port: 8443
    protocol: TCP
    targetPort: https
  selector:
    app.kubernetes.io/component: controller
    app.kubernetes.io/name: prometheus-operator
  sessionAffinity: None
  type: ClusterIP
status:
  loadBalancer: {}

Version-Release number of selected component (if applicable):
down grade from 4.6.0-0.nightly-2020-08-12-003456 to 4.5.0-0.nightly-2020-08-08-162221

How reproducible:
always

Steps to Reproduce:
1. down grade from 4.6.0-0.nightly-2020-08-12-003456 to 4.5.0-0.nightly-2020-08-08-162221
2.
3.

Actual results:
monitoring downgrade is blocked

Expected results:
no issue

Additional info: