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: Ibce5487c3018143b301935c9727007884f4d0974
Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/534224
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Commit-Queue: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@google.com>

go.mod

@@ -1,3 +1,3 @@
 module golang.org/x/xerrors
 
-go 1.17
+go 1.18

wrap_113_test.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.13
-// +build go1.13
 
 package xerrors_test