all: update go version to 1.18

Go versions 1.16 and 1.17 are long since unsupported per Go release
policy (https://go.dev/doc/devel/release#policy).

Updating go.mod's go statement to 1.18 makes it so that 'go mod tidy'
doesn't include checksums needed for the full module graph loaded by
Go 1.16¹ that were recently added in CL 507840.

It also makes go fix remove the now-obsolete // +build lines².

Done using cmd/go at go1.21rc2:

$ go get go@1.18
go: upgraded go 1.17 => 1.18
$ go mod tidy
$ go fix ./...
google/appengine_gen1.go: fixed buildtag
google/appengine_gen2_flex.go: fixed buildtag
internal/client_appengine.go: fixed buildtag

¹ https://go.dev/ref/mod#graph-pruning
² https://go.dev/doc/go1.18#go-build-lines

Change-Id: I6c6295adef1f5c64a196c2e66005763893efe5e7
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/507878
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cody Oss <codyoss@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

go.mod

@@ -1,6 +1,6 @@
 module golang.org/x/oauth2
 
-go 1.17
+go 1.18
 
 require (
 	cloud.google.com/go/compute/metadata v0.2.3