Remove deprecated nodeSelector label beta.kubernetes.io/os (#6970)

* Remove deprecated nodeSelector label beta.kubernetes.io/os

Problem:
The nodeSelector label beta.kubernetes.io/os in the CoreDNS deployment was deprecated in 1.14 and will likely be removed soon

Solution:
Change the nodeSelector to remove the beta

Signed-off-by: Dan Mills <evilhamsterman@gmail.com>

manifests/coredns.yaml

@@ -109,7 +109,7 @@ spec:
           operator: "Exists"
           effect: "NoSchedule"
       nodeSelector:
-        beta.kubernetes.io/os: linux
+        kubernetes.io/os: linux
       topologySpreadConstraints:
         - maxSkew: 1
           topologyKey: kubernetes.io/hostname