use embedded-kotlin to build plugins

[aSemy]

Gradle uses an embedded version of Kotlin. Gradle Plugins should not expose a different version of Kotlin as a dependency, to avoid causing conflicts with other Gradle Plugins.

https://docs.gradle.org/8.8/userguide/kotlin_dsl.html#sec:kotlin

Also, remove kotlin-stdlib-jdk8 - it is identical to kotlin-stdlib.

[Goooler]

I'm addressing a new fix to #66.

[liutikas]

Obsolete with https://github.com/androidx/gcp-gradle-build-cache/pull/72