[release-branch.go1.20] cmd/go: refuse to build Go 1.22 code

[release-branch.go1.20] cmd/go: refuse to build Go 1.22 code

With #60078 accepted, we expect Go 1.22 will have different
for loop semantics than Go 1.20 did. Once Go 1.22 is released,
Go 1.20 will be unsupported, but add a check anyway, just to
help catch some mistakes and usage of old Go toolchains
beyond their end-of-support.

Note that Go 1.20 can keep being used indefinitely with pre-Go 1.22 code.
This change only makes it refuse to build code that says it needs
Go 1.22 semantics, because Go 1.20 does not provide those.

For #60078.

Change-Id: I75118d6fbd0cc08a6bc309aca54c389a255ba7dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/518675
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>