Document the removal of youtube-dl from Debian 12.

Yt-dlp should instead be used.

Here's what I wrote in the NEWS.Debian file, for more info:

youtube-dl (2021.12.17-2) unstable; urgency=low

The youtube-dl project has been forked, and new development is happening under the yt-dlp project. We plan to completely remove the youtube-dl package after the Debian 12 (bookworm) release.

For the bookworm release, we're turning youtube-dl into an empty package that depends on yt-dlp. Most package dependencies in Debian are already aware of yt-dlp and will happily use it if available. However, USER SCRIPTS WILL BREAK AND NEED TO BE MANUALLY UPDATED BY ADMINS & USERS! We are not providing any kind of wrapper script, symlink, or wrapper module due to behavioral changes and namespace issues between the two programs.

In the vast majority of cases, you can simply replace "youtube-dl" with "yt-dlp" in scripts. However, if you were using a bunch of arguments, had config files, were using the downloaded files in other programs/scripts, or just want to revert behavior, be sure to read the yt-dlp man page section "Differences in default behavior". Passing the argument "--compat-options youtube-dl" to yt-dlp may be enough to revert behavior, but yt-dlp will still have some subtle differences.

Be sure to test your scripts thoroughly with yt-dlp!