Add JDK 20

[marchof]

Hi there, me again like every 6 month ;)

The first EA build of Java 20 is out: http://jdk.java.net/20/

Thanks for your very useful images!

[wglambert]

See docker-library/docs#2162 😢

[marchof]

Oh, that is sad. My primary use case was to test OpenJDK EA releases. Looks like I need find other images or create my own images.

[marchof]

Thanks @yosifkit !