cmd/go: add 'go version' statement in go.mod

cmd/go: add 'go version' statement in go.mod

We aren't planning to use this or advertise it much yet,
but having support for it now will make it easier to start
using in the future - older go commands will understand
what 'go 1.20' means and that they don't have go 1.20.

Fixes #23969.

Change-Id: I729130b2690d3c0b794b49201526b53de5093c45
Reviewed-on: https://go-review.googlesource.com/125940
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>