Use SPDX licences.

This is a followup to #3127.
At the time, SPDX licenses were pending approval by Google.

build.py

@@ -384,18 +384,7 @@ goog.provide('Blockly.utils.string');
 
  (Copyright \\d+ (Google LLC|Massachusetts Institute of Technology))
 ( All rights reserved.
-)?
- 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.
+)? SPDX-License-Identifier: Apache-2.0
 \\*/""")
     return re.sub(apache2, "", code)