rewrite samples to use Kotlin DSL for gradle

[robstoll]

Platform (all, jvm, js): all
Extension (none, kotlin 1.3): none

Code related feature

Currently, we use groovy in the samples. The Kotlin DSL is still somewhat RC I would say, not everything is smooth but I think it is good enough so that we can already use it in the samples projects

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[wordhou]

I can give this one a go this week.