go.mod: add go version directive
go.mod: add go version directive

(I mostly just wanted to make a trivial change to this repo to clear
out the failure cells for misconfigured builders on the landing page
of https://build.golang.org.)

Updates golang/go#32836

Change-Id: Ibcea7e75912ca377fd78cef37d8beb29ab8b83d3
Reviewed-on: https://go-review.googlesource.com/c/review/+/192557
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/review