Fix generating ui.pb.go on Ubuntu 18.04

[raboof]

This was broken because on Ubuntu 18.04, the '--go_opt' protoc option is not yet available.

Also clarify that for plugins=grpc to work, you need an older version of protoc-gen-go, which you can still get through go get github.com/golang/protobuf/protoc-gen-go.

https://stackoverflow.com/questions/62909028/unknown-flag-go-opt-while-regenerating-grpc-code