Description of problem:
When 

Version-Release number of selected component (if applicable):
4.11

How reproducible:
Always

Steps to Reproduce:
1. Enable user-defined AlertmanagerConfig in CMO config
kind: ConfigMap
apiVersion: v1
metadata:
  name: cluster-monitoring-config
  namespace: openshift-monitoring
data:
  config.yaml: |-
    alertmanagerMain:
      enableUserAlertmanagerConfig: true
2. Edit the alertmanager-main secret in the openshift-monitoring namespace with an OpsGenie receiver using the new entity field
receivers:
- name: opsgenie
  opsgenie_configs:
  - api_key: foo
    entity: bar
route:
  receiver: "opsgenie"


Actual results:
The PrometheusOperatorReconcileErrors alert is pending and there are errors in the prometheus-operator logs:

level=error ts=2022-05-30T12:29:27.451003887Z caller=klog.go:116 component=k8s_client_runtime func=ErrorDepth msg="Sync \"openshift-monitoring/main\" failed: provision alertmanager configuration: base config from Secret could not be parsed: yaml: unmarshal errors:\n line 32: field entity not found in type config.plain"


Expected results:
No error

Additional info:
The error doesn't happen when enableUserAlertmanagerConfig isn't true.