[ament_copyright] relax enforcement of Apache license in file header

ament_copyright is overzealous in enforcing its preferred header for Apache 2.0 license.

See http://www.apache.org/foundation/license-faq.html

* Use of the license *does not require* this header - only if it's likely that the source files somehow become separated from the license file.
* This check precludes the short variant suggested by Apache:
```
Copyright [yyyy] [name of copyright owner]
SPDX-License-Identifier: Apache-2.0
```
* Even the ASF doesn't use a copyright in the header: https://www.apache.org/legal/src-headers.html#headers