Update and clean up dependencies

This updates Compose to version beta5, Kotlin to the latest stable version and removes jackson-databind, as it was only carried over from briar-headless (for communication via JSON, I guess) and never used in Briar Desktop.

One visual change unfortunately comes with the new Compose version: IconButtons now have a minimum size of 48.dp "for better touch support". It looks a bit weird now, but I'd propose to address that in upcoming issue.

Closes #65 (closed)