The general answer is to Backup your library from MC20 and Restore it into MC21. That will get pretty much everything, including all of your media, custom fields, play counts, star ratings, etc .... and of course all of your Playlists, Playlist groups, views, etc.
File > Library > Backup library
File > Library > Restore library
Just take note of where the backup is made to, and restore from that location when you fire up MC21. The restore will overwrite everything in your current library in MC21.
Brian.