SPDX license format (#2667) (#2668)

* Switch to SPDX license format for all non-codegen files

* Add test to ensure all committed JS files have SPDX header

(cherry picked from commit 461f9b7f66)

Co-authored-by: Josh Mock <joshua.mock@elastic.co>

test/unit/helpers/bulk.test.ts

@@ -1,20 +1,6 @@
 /*
- * 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.
+ * Copyright Elasticsearch B.V. and contributors
+ * SPDX-License-Identifier: Apache-2.0
  */
 
 import FakeTimers from '@sinonjs/fake-timers'