go.mod: update to new compute/metadata module

This newly carved out module has 0 external dependencies which should
help simiplify many peoples dep trees.

Change-Id: I59fcc8517b92fea0142fdee1be12076c4ece3e61
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/446075
Auto-Submit: Cody Oss <codyoss@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Cody Oss <codyoss@google.com>

go.mod

@@ -3,7 +3,7 @@ module golang.org/x/oauth2
 go 1.17
 
 require (
-	cloud.google.com/go/compute v1.7.0
+	cloud.google.com/go/compute/metadata v0.2.0
 	github.com/google/go-cmp v0.5.8
 	golang.org/x/net v0.1.0
 	google.golang.org/appengine v1.6.7