OpenShift 4 upgrade stuck: The cluster operator storage has not yet successfully rolled out

Issue

• The clusterversion has an error regarding the Cluster Operator storage:

  $ oc get clusterversion
  NAME      VERSION   AVAILABLE   PROGRESSING   SINCE   STATUS
  version   4.3.31    True        True          143m    Unable to apply 4.3.33: the cluster operator storage has not yet successfully rolled out
  

• The Cluster Operator storage has the following error message:

  $ oc describe co storage
  [...]
  Message:               Unable to update cluster as v1alpha1 version of volumesnapshotclasses.snapshot.storage.k8s.io, volumesnapshotcontents.snapshot.storage.k8s.io, volumesnapshots.snapshot.storage.k8s.iois detected. Remove these CRDs to allow the upgrade to proceed.
  [...]