go.mod: update stale dependencies

Invokes go get -u -v ./... to try to get rid of
stale dependencies cloud.google.com/[email protected] (released in December 2018)
and that dependency unfortunately imported a stale
version of go.opencensus.io that then transitively
imports another that has a vulnerability.

Change-Id: I0a520e8f979ff3ddcd6197b234cdc0f4154bb735
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/245517
Run-TryBot: Emmanuel Odeke <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: odeke-em

go.mod

@@ -3,8 +3,7 @@ module golang.org/x/oauth2
 go 1.11
 
 require (
-	cloud.google.com/go v0.34.0
-	golang.org/x/net v0.0.0-20190108225652-1e06a53dbb7e
-	golang.org/x/sync v0.0.0-20181221193216-37e7f081c4d4 // indirect
-	google.golang.org/appengine v1.4.0
+	cloud.google.com/go v0.65.0
+	golang.org/x/net v0.0.0-20200822124328-c89045814202
+	google.golang.org/appengine v1.6.6
 )