Introduce new URL scheme for determining distribution version

[marc0der]

• Upgrade to Gradle 6.9.3
• Introduce Kotlin to build
• Accommodate new binary version URL scheme
• Maintain backward compatibility
• Rewrite gnarly BinaryVersionHandler in Kotlin(!)
• Introduce the concept of a beta native CLI version(?)

The last point is contentious, and I'm still in two minds about it. Do we want to roll with a simple straight-to-prod approach with the native CLI instead? To me, this almost feels like a better approach.