The text at the top of each source file was already specifying 2.0-or-later,
but this wasn't clearly reflected in the full-text license in COPYING,
which was the old 2.0 license, which can be interpreted as 2.0-only.

Note that the license of `liblz4`, the library in `lib/` directory,
is unchanged, and remains the more permissive `BSD 2-clause`.