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: I3817780240e082ba076e2c6c9cd45fd3837c3c1d
Reviewed-on: https://go-review.googlesource.com/c/playground/+/534736
Reviewed-by: Robert Findley <rfindley@google.com>
Commit-Queue: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>

client.go

diff --git a/client.go b/client.go
index c49c121..f7a8795 100644
--- a/client.go
+++ b/client.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 // Copyright 2014 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style

go.mod

diff --git a/go.mod b/go.mod
index 34b6b12..84a7d3f 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module golang.org/x/playground
 
-go 1.17
+go 1.18
 
 require (
 	cloud.google.com/go/compute/metadata v0.2.3