SPDX license format (#2667)

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

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

test/integration/reporter.js

@@ -1,3 +1,8 @@
+/*
+ * Copyright Elasticsearch B.V. and contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
 'use strict'
 
 const assert = require('node:assert')