PTP operator: Use priority class node critical

Hi ! The following test case is failing when executing OCP conformance tests against OCP4.12 on a SNO bare metal. Our testing effort is motivated by https://issues.redhat.com/browse/CNF-6706:

[sig-arch] Managed cluster should ensure platform components have system-priority class associated [Suite:openshift/conformance/parallel] is failing because it has checked that ptp-operator has an invalid priority class (currently "" vs an expected one which should be 'openshift-user-critical' or begin with 'system-').