lavr: deprecate the entire library

Deprecate the entire library. Merged years ago to provide compatibility
with Libav, it remained unmaintained by the FFmpeg project and duplicated
functionality provided by libswresample.

In order to improve consistency and reduce attack surface, as well as to ease
burden on maintainers, it has been deprecated. Users of this library are asked
to migrate to libswresample, which, as well as providing more functionality,
is faster and has higher accuracy.

Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>