Add migration documentation on the correct procedure to move from runc to crun for born in 4.17 and below clusters that maintained the runc runtime when upgrading to 4.18+.

We only mention this in the 4.18 release notes but not a procedure:
https://docs.redhat.com/en/documentation/openshift_container_platform/4.18/html-single/release_notes/index#ocp-release-notes-nodes-crun-default_release-notes

The upgrade creates a new machineconfig for each pool like: 00-override-worker-generated-crio-default-container-runtime and a configmap crio-default-container-runtime in openshift-machine-config-operator.

Should the MC and CM be removed to take the cluster back to the crun default or should customers apply a ContainerRuntimeConfig to explicitly set the default to crun to override the MC.

4.18+

Always

Unclear what action to take to move from runc to crun.