add boilerplate.generatego.txt and let all code-generators use it

remove YEAR field in boilerplate under staging

add Do Not Edit for types_swagger_doc and add apiextensions-apiserver's example in codegen.sh

hack/boilerplate/boilerplate.generatego.txt

@@ -0,0 +1,16 @@
+/*
+Copyright The Kubernetes Authors.
+
+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.
+*/
+