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.

buildSrc/build.gradle.kts

@@ -0,0 +1,27 @@
+plugins {
+    `kotlin-dsl`
+    checkstyle
+    id("net.kyori.indra.publishing") version "2.0.6"
+}
+
+repositories {
+    mavenCentral()
+    maven("https://plugins.gradle.org/m2")
+}
+
+gradlePlugin {
+    plugins {
+        register("set-manifest-impl-version") {
+            id = "set-manifest-impl-version"
+            implementationClass = "com.velocitypowered.script.SetManifestImplVersionPlugin"
+        }
+        register("velocity-checkstyle") {
+            id = "velocity-checkstyle"
+            implementationClass = "com.velocitypowered.script.VelocityCheckstylePlugin"
+        }
+        register("velocity-publish") {
+            id = "velocity-publish"
+            implementationClass = "com.velocitypowered.script.VelocityPublishPlugin"
+        }
+    }
+}