Prepare to drop Java 8 support as Gradle moves towards 17

[thecatcore]

Gradle 9 will drop run support for Java 8 in favor of Java 17.
Gradle 8.9 already warns about it which breaks compilation tests on develop branch.

[wagyourtail]

will probably just drop java 8 in tests, for now, at least for a bit I'll try to make unimined support gradle 8 still.
as for dev'ing legacy stuff, this should still be possible with *toolchains* (or the release flag on java compile, but toolchains are cooler)