protoc-gen-go/generator deprecation

[twifkak]

When building cmd/amppkg with -tags dns01 and running, we get the following warning msg on stdout:

WARNING: Package "github.com/golang/protobuf/protoc-gen-go/generator" is deprecated.
        A future release of golang/protobuf will delete this package,
        which has long been excluded from the compatibility promise.

AFAIK, we don't use this directly, so it's some indirect usage (lego?).