Use kotlinx-datetime

[BartArys]

The madlads over at Jetbrains just released another banger library that brings Java's DateTime API to kotlin-multiplatform. Since our project's only real remaining dependency on the Java stdlib is the datetime API we can use this to yeet that stuff out of there and then realize that we just imported megabytes of typealiases. It'll be grand.

Note that kotlinx-datetime does not typealias java.time. As such, we should leave the external API using java time intact, simply providing kotlin's version as an alternative, and remove java time as an internal dependency.

[Tmpod]

The library is still lacking a few features we use, specifically better formatters for formatting and parsing dates in ISO+Offset.

Nevertheless, I'm starting to port whatever I can to the new Instant class and so on. I'm working on 0.7.x and will soon make a WIP PR as Bart asked for in the Discord server.

[DRSchlaubi]

There actually is a PR for Kotlin/kotlinx-datetime#56 which is their issue for this bug now: Kotlin/kotlinx-datetime#107
They also pushed kx.ser support to master