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: Iffde5a8298df8aa6c9ae17c73016e0669bba725d
Reviewed-on: https://go-review.googlesource.com/c/tour/+/534735
Run-TryBot: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Commit-Queue: Dmitri Shuralyov <dmitshur@google.com>

go.mod

@@ -1,3 +1,3 @@
 module golang.org/x/tour
 
-go 1.16
+go 1.18