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/tools/leaderelection/OWNERS

@@ -0,0 +1,15 @@
+# See the OWNERS docs at https://go.k8s.io/owners
+
+approvers:
+- mikedanese
+- timothysc
+reviewers:
+- wojtek-t
+- deads2k
+- mikedanese
+- gmarek
+- eparis
+- timothysc
+- ingvagabund
+- resouer
+- goltermann