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>