Update license header (#5643) (#5645)

Co-authored-by: Steve Gordon <sgordon@hotmail.co.uk>

Author: github-actions[bot]

src/ApiGenerator/Views/GeneratorNotice.cshtml

@@ -1,5 +1,24 @@
 @using ApiGenerator
 @inherits CodeTemplatePage<object>
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you 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.
+ */
+
 // ███╗   ██╗ ██████╗ ████████╗██╗ ██████╗███████╗
 // ████╗  ██║██╔═══██╗╚══██╔══╝██║██╔════╝██╔════╝
 // ██╔██╗ ██║██║   ██║   ██║   ██║██║     █████╗