Monitoring operator is degraded with message "updates to statefulset spec for fields are forbidden"

Issue

• Attempting to run an upgrade but it's stuck with the Prometheus operators in Degraded / Progressing

• The prometheus-operator logs show the following:

  2021-11-02T17:51:24.247186465Z level=info ts=2021-11-02T17:51:24.246371145Z caller=operator.go:804 component=alertmanageroperator key=openshift-monitoring/main msg="recreating AlertManager StatefulSet because the update operation wasn't possible" reason="Forbidden: updates to statefulset spec for fields other than 'replicas', 'template', 'updateStrategy' and 'minReadySeconds' are forbidden"