Use SPDX license header

E.g. like this (in particular, without a year):

```
/*
 * SPDX-License-Identifier: Apache-2.0
 *
 * Copyright The original authors
 *
 * Licensed under the Apache Software License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
 */
```