Support Java 8

[simolus3]

This:

1. Removes the use of Java toolchains for compiling code. These are "rarely a good idea" because newer JDKs are perfectly capable of emitting code for older JVMs (this is especially true for us since we only use Kotlin anyway).
2. Uses Java 8 as a target for the JVM-specific KMP targets. We're still targeting Java 17 for Android, like before.
3. Reformats code so that a clean ./gradlew build works. I'll look into why formatting isn't applied on the CI, but that's less time-critical.
4. Upgrades stately to a version that supports Java 8.
5. Run JVM tests for the core project on Java 8, in addition to the JVM used to build the project.

[DominicGBauer]

Assuming you've tested this in the demo LGTM