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/client-go/util/keyutil/OWNERS

@@ -0,0 +1,7 @@
+approvers:
+- sig-auth-certificates-approvers
+reviewers:
+- sig-auth-certificates-reviewers
+labels:
+- sig/auth
+