Although we originally thought this would be a useful way to do this, it
doesn't enforce the `go.mod` constraints that we'd hoped it would, as
Renovate could bump the `go.mod`'s `go` directive, and it means that we
have a more explicit, and less magic, setup.

Additionally, we need to quote the versions as `1.20` gets interpreted
by YAML as `1.2` 🙃