Watch HA configmap via name instead of lables to reduce pressure on APIserver

As FLINK-24819 described, the k8s API server would receive more pressure when HA is enabled, due to the configmap watching being achieved via filter with labels instead of just querying the configmap name. This could be done after FLINK-24038, which reduced the number of configmaps to only one as <clusterId>-cluster-config-map.

This ticket would not touch <clusterId>-<jobId>-config-map, which stores the checkpoint information, as that configmap is directly accessed by JM and not watched by taskmanagers.