[BACKEND] Remove old format for video_tags, music_tags, download_strategy (#899)
Now that sufficient time has passed, we can formally deprecate the following:
- https://ytdl-sub.readthedocs.io/en/latest/deprecation_notices.html#video-tags
- https://ytdl-sub.readthedocs.io/en/latest/deprecation_notices.html#music-tags
- usage of
download_strategy(simply remove it if you're getting an error about it)