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/code-generator/OWNERS

@@ -1,13 +0,0 @@
-# See the OWNERS docs at https://go.k8s.io/owners
-
-approvers:
-- lavalamp
-- wojtek-t
-- sttts
-reviewers:
-- lavalamp
-- wojtek-t
-- sttts
-labels:
-- sig/api-machinery
-- area/code-generation