Change the licence holder to The Rust Project Developers

The copyright of this project belongs to The Rust Project Developers,
who are mentioned in AUTHORS.txt.
auto merge of #13391 : smesseim/rust/apache-copyright, r=alexcrichton

The years of copyright and the name of the copyright holder were not
present in the notice.

The Apache license was added to the project in 2012, so 2012 is the
starting year. The copyright holder is the Mozilla Foundation (taken
from the MIT license).