This mirrors the change in the main OpenRS2 repository. I think the
natives actually work all the way back to Java 6 (or maybe even
earlier). However, we're only routinely testing the build against JDK11
(plus legacy Java 6 on macOS).
I haven't tested on 7 (and I'm not sure if the jawt shared lib files are
even in the correct place), so I think it's better to require the
version I've been testing on.
This is also consistent with the requirement for the main openrs2
project.
Graham