Request: OpenJDK 15

[lorenzleutgeb]

I am missing version 15 of the openjdk package, see https://jdk.java.net/15/

[jerith666]

cc folks who've worked on JDK updates recently: @asbachb @bachp @edwtjo @Ma27 @petabyteboy

[raboof]

I started on adding openjdk15 in #100403 - it's usable but not complete. I have to move on to other tasks right now though, so feel free to take over.