canary: Add priority class to canary daemonset
OpenShift components are required to specify a PriorityClassName for daemonsets/deployments. This commit ensures that the Canary daemonset has the `system-cluster-critical` priority class set. assets/canary/daemonset.yaml: Set the `priorityClassName` field to be `system-cluster-critical`. pkg/operator/controller/canary/daemonset.go: Update the Canary daemonset when `priorityClassName` changes. pkg/operator/controller/canary/daemonset_test.go: Add a test to ensure that `priorityClassName` is set correctly by `desiredCanaryDaemonSet`. Add a test to verify that the Canary daemonset is updated when `priorityClassName` is mutated. This commit is in support of Bug 1954892.