all: update go directive to 1.18

Done with:

go get go@1.18
go mod tidy
go fix ./...

Using go1.21.3.

Also update code generators to use only the new go:build lines,
not the old +build ones.

For golang/go#60268.

Change-Id: I6aabc42efb6ab3329981100e1db2263aac5e92a6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/534222
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

execabs/execabs_go118.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !go1.19
-// +build !go1.19
 
 package execabs
 

execabs/execabs_go119.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.19
-// +build go1.19
 
 package execabs