Update track blurb

I think that

> Kotlin is a pragmatic programming language for JVM and Android that combines OO and functional features and is focused on interoperability, safety, clarity and tooling support.

is a bit outdated. At least, Kotlin has [Multiplatform](https://kotlinlang.org/docs/reference/multiplatform.html) targets.

Search the Kotlin track repository for the quoted track description and inspect the surrounding track metadata. Update the blurb to reflect Kotlin Multiplatform, then confirm the old wording is no longer present and the documentation remains valid.

Written by the indexing model from the issue text.