[7.0] Update testing setup

[PaintNinja]

This PR updates to jUnit 6 and newer Google Compile Testing, as well as some minor cleanup.

https://github.com/junit-team/junit-framework/wiki/Upgrading-to-JUnit-6.0

The jUnit 6 upgrade was far easier than expected - they removed long-deprecated APIs, bumped the min Java to 17, switched to jSpecify, etc. All those requirements were already met so ended up being a case of bumping some numbers and confirming nothing broke.

Google in their infinite wisdom says that com.google.testing.compile 0.22.0+ needs JVM flags manually added to export JDK internals to everyone and doesn't explain why, but seems to be unnecessary for this project as far as I can tell because it works same as usual without them. The auto-value transitive dep manual version bump workaround is no longer needed though which is nice.