multimedia/ffmpeg2theora: Deprecate and set expiration date to 2022-06-30

Theora itself hasn't seen development apart from minor fixes in over 10
years and have been superseded by AV1/H.264/H.265/VP8 and VP9 (depending
on view regarding royalties) and by now this port has outlived its
purpose. Users still wanting to encode in this format can still use
ffmpeg as-is.

PR: 263528
Approved by: wg (maintainer timeout, 2+ weeks)