cmd/go/internal/modload: set the default GoVersion in a single location

For #46141
Updates #36460

Change-Id: Ie4c13c73a451650d1e8abb8e5cebfc30d0a71a70
Reviewed-on: https://go-review.googlesource.com/c/go/+/321070


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>