go.mod: update kubernetes to v1.20.6

Includes updated protos with the gogo/protobuf fixes for CVE-2021-3121
(https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3121)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

go.mod

@@ -55,12 +55,12 @@ require (
 	golang.org/x/sys v0.0.0-20210324051608-47abb6519492
 	google.golang.org/grpc v1.33.2
 	gotest.tools/v3 v3.0.3
-	k8s.io/api v0.20.4
-	k8s.io/apimachinery v0.20.4
-	k8s.io/apiserver v0.20.4
-	k8s.io/client-go v0.20.4
-	k8s.io/component-base v0.20.4
-	k8s.io/cri-api v0.20.4
+	k8s.io/api v0.20.6
+	k8s.io/apimachinery v0.20.6
+	k8s.io/apiserver v0.20.6
+	k8s.io/client-go v0.20.6
+	k8s.io/component-base v0.20.6
+	k8s.io/cri-api v0.20.6
 	k8s.io/klog/v2 v2.4.0
 	k8s.io/utils v0.0.0-20201110183641-67b214c5f920
 )