Unify Gradle build styles

[marcprux]

In some places, the examples use the modern (and preferred) build.gradle.kts kotlinscript syntax, and in others they use the older build.gradle groovy syntax (like in swift-java-weather-app/weather-lib/build.gradle and hello-swift-java/hashing-lib/build.gradle).

It would be good to standardize on a single style for the sample build scripts, not least because it would enable us to share some of the logic with the root swift-android.gradle.kts.

This could also serve as a playground for an eventual set of official recommendations for how to construct Gradle projects that use Swift without necessarily relying on any external plugins.

[finagolfin]

@madsodgaard, were these scripts automatically generated to some extent, hence the different styles Marc claims?