games/prismlauncher: Build with java 7 compatibility if build with jdk < 20

Java 20 and above has dropped binary compatibility with Java 7.
When the Prism Launcher is compiled with Java 20+, the sources are
patched to be binary compatible with Java 8 instead of Java 7.
This may cause issues with very old Minecraft versions and/or older
Forge versions: https://github.com/PolyMC/PolyMC/issues/515.
If you have problems, install an older Java compiler and recompile
games/Prismlauncher.

Approved by: Alexander88207@protonmail.com (maintainer, via Discord)