all: update go directive to 1.18

Done with:

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

Using go1.21.3.

For golang/go#60268.

Change-Id: Ic6a53c8e217966f5f95b3a968b25ef123b4b99cf
Reviewed-on: https://go-review.googlesource.com/c/term/+/534475
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

go.mod

@@ -1,5 +1,5 @@
 module golang.org/x/term
 
-go 1.17
+go 1.18
 
 require golang.org/x/sys v0.13.0

term_unix.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris || zos
-// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris zos
 
 package term
 

term_unix_bsd.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build darwin || dragonfly || freebsd || netbsd || openbsd
-// +build darwin dragonfly freebsd netbsd openbsd
 
 package term
 

term_unix_other.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build aix || linux || solaris || zos
-// +build aix linux solaris zos
 
 package term
 

term_unsupported.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !aix && !darwin && !dragonfly && !freebsd && !linux && !netbsd && !openbsd && !zos && !windows && !solaris && !plan9
-// +build !aix,!darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd,!zos,!windows,!solaris,!plan9
 
 package term