Update to JUnit5, Update to J17 and J21 for tests

[NotStirred]

I switched from google truth to assertj, as truth depends heavily on junit4. No code changes thankfully, only imports (apis we use are identical)

Only thing of note: QuickMathTest#testAbs_1 was previously incorrect (and now fails, relevant part is commented out)

Probably best not to squash as the two commits are entirely separate

[NotStirred]

I moved most things into the subproject closure so it should be much easier to understand. Everything now has junit tests and jacoco coverage enabled.

[leMaik]

Maybe we can keep the launcher compatible with Java 8, just in case. At least the part that checks and updates Java.

[NotStirred]

hrm, in that case it may be best to specify in each project separately, as the launcher depends on lib too