9.4.

$ oc create configmap trusted-ca -n cert-manager

$ oc label cm trusted-ca config.openshift.io/inject-trusted-cabundle=true -n cert-manager

$ oc -n cert-manager-operator patch subscription openshift-cert-manager-operator --type='merge' -p '{"spec":{"config":{"env":[{"name":"TRUSTED_CA_CONFIGMAP_NAME","value":"trusted-ca"}]}}}'

$ oc rollout status deployment/cert-manager-operator-controller-manager -n cert-manager-operator && \
oc rollout status deployment/cert-manager -n cert-manager && \
oc rollout status deployment/cert-manager-webhook -n cert-manager && \
oc rollout status deployment/cert-manager-cainjector -n cert-manager

deployment "cert-manager-operator-controller-manager" successfully rolled out
deployment "cert-manager" successfully rolled out
deployment "cert-manager-webhook" successfully rolled out
deployment "cert-manager-cainjector" successfully rolled out

$ oc get deployment cert-manager -n cert-manager -o=jsonpath={.spec.template.spec.'containers[0].volumeMounts'}

[{"mountPath":"/etc/pki/tls/certs/cert-manager-tls-ca-bundle.crt","name":"trusted-ca","subPath":"ca-bundle.crt"}]

$ oc get deployment cert-manager -n cert-manager -o=jsonpath={.spec.template.spec.volumes}

[{"configMap":{"defaultMode":420,"name":"trusted-ca"},"name":"trusted-ca"}]