Update go.mod to 1.20

The latest go.mod pins go version to 1.16, but given it vendors k8s v1.23 where used go 1.19, it's better to bump go version to 1.19+ as well.

Otherwise, running `go mod vendor` would fail with:

weih@m1max:~/go/src/github.com/apache/yunikorn-k8shim|master ⇒  go version
go version go1.16.15 darwin/arm64
weih@m1max:~/go/src/github.com/apache/yunikorn-k8shim|master ⇒  go mod vendor
internal error: failed to find embedded files of go.uber.org/zap: //go:build comment without // +build comment

Switching to go1.19 works fine:

weih@m1max:~/go/src/github.com/apache/yunikorn-k8shim|master ⇒  go version
go version go1.19.8 darwin/arm64
weih@m1max:~/go/src/github.com/apache/yunikorn-k8shim|master ⇒  go mod vendor