replace "dep" with "go mod"

A simple "go mod init && go mod tidy && go mod vendor" was enough and
mostly seems to have picked the same revisions of the dependencies as
before. Notable exceptions are klog and gogo/protobuf.

vendor/k8s.io/kube-openapi/pkg/util/proto/OWNERS

@@ -0,0 +1,2 @@
+approvers:
+- apelisse