[VOL-1866] Changed module dependency to v12.0.0 of k8s client-go and v1.15.4 of k8s api/apimachinery in sync with other voltha components

Had to use pseudo-version corresponding to v12.0.0 of k8s client-go
because golang proxy is no longer serving the modules not complying
to Semantic Import Versioning rules including client-go v12.0.0.
Refer to https://github.com/kubernetes/client-go/issues/631 and
https://github.com/golang/go/issues/33558

Change-Id: I2e558bab7f0702f230761319eb5392a7d0532ea3
diff --git a/vendor/k8s.io/api/settings/v1alpha1/types.go b/vendor/k8s.io/api/settings/v1alpha1/types.go
index 506aacf..8cc99d4 100644
--- a/vendor/k8s.io/api/settings/v1alpha1/types.go
+++ b/vendor/k8s.io/api/settings/v1alpha1/types.go
@@ -61,7 +61,7 @@
 type PodPresetList struct {
 	metav1.TypeMeta `json:",inline"`
 	// Standard list metadata.
-	// More info: https://git.k8s.io/community/contributors/devel/api-conventions.md#metadata
+	// More info: https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#metadata
 	// +optional
 	metav1.ListMeta `json:"metadata,omitempty" protobuf:"bytes,1,opt,name=metadata"`