Remove Groovy DSL syntax from Gradle plugin

[MiSikora]

:bulb: Motivation

Groovy DSL interferes in strange ways with the Gradle APIs. It does not allow nesting missing methods, and sometimes it catches functions that accept non-primitive arguments (like Project). Plus, it creates a mismatch between Kotlin scripts and Groovy scripts.

:pencil: Checklist

• [x] I updated the changelog.
• [x] I updated the documentation.
• [x] I updated the sample.

:crystal_ball: Next steps

N/A