switch from youtube-dl to yt-dlp

After looking at the GitHub presence of youtube-dl and reading through the discussions on open issues, I concluded that the project is unmaintained at present. The last version was released on 2021-06-06. yt-dlp seems to be the way to go. I updated my little helpers for Windows and Mac to use it. Downloads links are on the original pages: Note to myself: it is time to move from tar and zip to a proper public repository soon.