General dependency update

[chris-hatton]

I am interested in taking on issue #89, to support kotlinx.serialization.

In preparation, I checked out master and observed that dependencies were generally somewhat out-of-date. I've updated them here before adding support for the latest kotlinx.serialization version.

A couple of minor changes arose to account for the new dependencies:

1. Dokka now prefers to expose a task-per-output-format, than have a single output format selected. I imagine this is to support output of multiple formats. The way the task is acquired for the kotlinDocJar task then had to change.
2. Kotlin Poet now supports multple 'simpleName's; I'm not sure why to be honest but fixing our usage was a simple case of removing the single argument label.

Unit tests passing locally ✅

[cjbooms]

👍

[cjbooms]

Thanks @chris-hatton