knative Pods are blocking cluster upgrade

Issue

• When trying to upgrade an OpenShift Container Platform cluster, the upgrade is stuck due to knative-serving and knative-eventing Pods not being deleted.
• The PodDisruptionBudgets for knative Deployments are blocking the upgrade, why is that?

• In the openshift-machine-config-operator Pod the following errors can be seen:

  I0817 10:53:15.163969    1620 daemon.go:334] evicting pod knative-serving/webhook-55f48cd87-8htvf
  E0817 10:53:15.176818    1620 daemon.go:334] error when evicting pods/"webhook-55f48cd87-8htvf" -n "knative-serving" (will retry after 5s): Cannot evict pod as it would violate the pod's disruption budget.
  I0817 10:53:20.162832    1620 daemon.go:334] evicting pod knative-eventing/eventing-webhook-8dcf8c585-xqrkl
  I0817 10:53:20.177187    1620 daemon.go:334] evicting pod knative-serving/webhook-55f48cd87-8htvf
  E0817 10:53:20.178274    1620 daemon.go:334] error when evicting pods/"eventing-webhook-8dcf8c585-xqrkl" -n "knative-eventing" (will retry after 5s): Cannot evict pod as it would violate the pod's disruption budget.
  E0817 10:53:20.190681    1620 daemon.go:334] error when evicting pods/"webhook-55f48cd87-8htvf" -n "knative-serving" (will retry after 5s): Cannot evict pod as it would violate the pod's disruption budget.
  I0817 10:53:25.178461    1620 daemon.go:334] evicting pod knative-eventing/eventing-webhook-8dcf8c585-xqrkl
  I0817 10:53:25.191681    1620 daemon.go:334] evicting pod knative-serving/webhook-55f48cd87-8htvf
  I0817 10:53:25.191809    1620 drain.go:92] Draining failed with: [error when evicting pods/"eventing-webhook-8dcf8c585-xqrkl" -n "knative-eventing": global timeout reached: 1m30s, error when evicting pods/"webhook-55f48cd87-8htvf" -n "knative-serving": global timeout reached: 1m30s], retrying