* Update Java and fix AppVeyor
Target Java 16
Run CI using Java 17
Use provided Maven installation instead of relying on mirror link
Remove Java 7 memory limitation settings
* Fix Jitpack build for Java > 8
Jitpack uses Java 8 by default. While their (very minimal) explanation of settings suggests specifying `jdk: [ openjdk17 ]` would be enough, it does not appear to work. Not sure if that's just an issue with their image not including JDK17 yet or something, but this does function.
