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/SECURITY_CONTACTS

@@ -1,17 +0,0 @@
-# Defined below are the security contacts for this repo.
-#
-# They are the contact point for the Product Security Committee to reach out
-# to for triaging and handling of incoming issues.
-#
-# The below names agree to abide by the
-# [Embargo Policy](https://git.k8s.io/security/private-distributors-list.md#embargo-policy)
-# and will be removed and replaced if they violate that agreement.
-#
-# DO NOT REPORT SECURITY VULNERABILITIES DIRECTLY TO THESE NAMES, FOLLOW THE
-# INSTRUCTIONS AT https://kubernetes.io/security/
-
-cjcullen
-jessfraz
-liggitt
-philips
-tallclair