Convert Velocity buildscripts to Kotlin DSL (#918)

Spiritually indebted to #518 and @alexstaeding.

There's a minor break - we're going up to 3.2.0-SNAPSHOT as the API now compiles against Java 11. But this is more academic in practice.

gradle.properties

@@ -0,0 +1,26 @@
+# API dependencies.
+gsonVersion=2.9.0
+junitVersion=5.9.0
+slf4jVersion=1.7.30
+adventureVersion=4.12.0
+guavaVersion=25.1-jre
+checkerFrameworkVersion=3.6.1
+configurateVersion=3.7.3
+guiceVersion=5.0.1
+
+# Proxy dependencies.
+log4jVersion=2.19.0
+nettyVersion=4.1.86.Final
+flareVersion=2.0.1
+asyncHttpClientVersion=2.12.3
+bstatsVersion=2.2.1
+caffeineVersion=3.1.1
+lmbdaVersion=2.0.0
+nightConfigVersion=3.6.4
+completableFuturesVersion=0.3.5
+adventureFacetVersion=4.1.2
+fastutilVersion=8.5.8
+disruptorVersion=3.4.4
+jansiVersion=3.21.0
+terminalConsoleAppenderVersion=1.3.0
+joptSimpleVersion=5.0.4