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/pkg/apis/clientauthentication/OWNERS

@@ -0,0 +1,9 @@
+# See the OWNERS docs at https://go.k8s.io/owners
+
+# approval on api packages bubbles to api-approvers
+reviewers:
+- sig-auth-authenticators-approvers
+- sig-auth-authenticators-reviewers
+labels:
+- sig/auth
+