Update folders module to Terraform 0.13

With this commit the folders module (now called simply 'folder') only
creates a single google_folder resource. Support for creating multiple
folders is no longer needed since Terraform 0.13 added for_each support
to modules.

modules/folder/versions.tf

@@ -0,0 +1,19 @@
+/**
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+terraform {
+  required_version = ">= 0.13.0"
+}