Update used Go version to 1.16.x

We're currently on 1.13.8, while the recent stable version of Go is 1.16.0. We're - AGAIN - on an unsupported version of Go.

Since we're not blocked (or rather - we don't know about any blocker) from bumping the version up, as we were with Go 1.8, let's just do this. And see if all is OK 🙂

This is a successor of #27208 (closed).