How problematic can it be to learn Java with the old version? The course is based on Java 12 and now the latest version is Java 20.
Sure there are new things in Java 20 but many companies are still running Java 8 so you can still use that as a base for your learning.
The best you could do is find out what new between Java 12 and Java 20 and decide if you can manage to learn it on your own (Or choose an up to date course from the beginning).
It will be no problem at all because Mosh teaches you things how Java used to do it and then teaches you the new ways Java does it. So Mosh thought of you XD