protoc-gen-go/generator deprecation

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?).

Reproduce the warning by building and running cmd/amppkg with the dns01 tag, then trace which dependency brings in github.com/golang/protobuf/protoc-gen-go/generator, possibly through lego. Done means the indirect dependency is updated or removed and the warning no longer appears on stdout.

Written by the indexing model from the issue text.