gcs: reduce oauth scope requested as suggested by Google

As part of getting the rclone oauth consent screen approved by Google,
it came up that the scope in use by the gcs backend was too large.

This change reduces it to the minimum scope which still allows rclone
to work correctly.

Old scope: https://www.googleapis.com/auth/devstorage.full_control
New scope: https://www.googleapis.com/auth/devstorage.read_write

backend/googlecloudstorage/googlecloudstorage.go

@@ -61,7 +61,7 @@ const (
 var (
 	// Description of how to auth for this app
 	storageConfig = &oauth2.Config{
-		Scopes:       []string{storage.DevstorageFullControlScope},
+		Scopes:       []string{storage.DevstorageReadWriteScope},
 		Endpoint:     google.Endpoint,
 		ClientID:     rcloneClientID,
 		ClientSecret: obscure.MustReveal(rcloneEncryptedClientSecret),