[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/events/v1beta1/generated.proto b/vendor/k8s.io/api/events/v1beta1/generated.proto
index b3e565e..04eacbb 100644
--- a/vendor/k8s.io/api/events/v1beta1/generated.proto
+++ b/vendor/k8s.io/api/events/v1beta1/generated.proto
@@ -116,6 +116,7 @@
   optional k8s.io.apimachinery.pkg.apis.meta.v1.MicroTime lastObservedTime = 2;
 
   // Information whether this series is ongoing or finished.
+  // Deprecated. Planned removal for 1.18
   optional string state = 3;
 }