Upgrade Kotlin and Gradle

[LarsSven]

This is a followup of #534 (review)

It upgrades Kotlin and Gradle to a newer version.

These are some pretty major upgrades so I do recommend also locally testing it before approving/merging.

[LarsSven]

The current CI failure seems to be a random failure that probably needs to be rerun?

[TimvdLippe]

I have rerun it on CI a couple of times and it appears to consistently fail. Please investigate the failure and let us know why it is failing.

[LarsSven]

The doReturn_withGenericIntReturnType_on test specifically fails on the upgrade from Kotlin 1.9 to 2.0 (2.0.0 is the first version that starts failing). It is not fully clear to me why it starts failing, especially because while on does not work, onGeneric does work fine, and the generated error message by mockito-kotlin also suggests that you may need to use onGeneric in this case.

I am not deep enough in this codebase to understand what on and onGeneric do specifically and why one would fail with an NPE and the other wouldn't. Maybe 2.0 changes something in how they enforce nullability in Kotlin?

[LarsSven]

I had another look at it but it almost feel like maybe the test is not correct. Why did on work on generic methods to begin with, aren't you supposed to always use onGeneric for generic methods?