all: update go version to 1.17

This enables module graph pruning
(https://go.dev/ref/mod#graph-pruning) for this module, pruning out
unncessary dependencies for other modules at 'go 1.17' or higher that
require this one.

Notably, this may prune out a great many transitive dependencies of
cloud.google.com/go.

Change-Id: Ide1030fc0d308dc768447c4e47c371dfdca6387c
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/415394
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

go.mod

@@ -1,9 +1,15 @@
 module golang.org/x/oauth2
 
-go 1.15
+go 1.17
 
 require (
 	cloud.google.com/go v0.65.0
+	github.com/google/go-cmp v0.5.1
 	golang.org/x/net v0.0.0-20220127200216-cd36cc0744dd
 	google.golang.org/appengine v1.6.6
 )
+
+require (
+	github.com/golang/protobuf v1.4.2 // indirect
+	google.golang.org/protobuf v1.25.0 // indirect
+)