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.

api/src/ap/java/com/velocitypowered/api/plugin/ap/PluginAnnotationProcessor.java

@@ -27,6 +27,9 @@ import javax.tools.Diagnostic;
 import javax.tools.FileObject;
 import javax.tools.StandardLocation;
 
+/**
+ * Annotation processor for Velocity.
+ */
 @SupportedAnnotationTypes({"com.velocitypowered.api.plugin.Plugin"})
 public class PluginAnnotationProcessor extends AbstractProcessor {
 

api/src/ap/java/com/velocitypowered/api/plugin/ap/SerializedPluginDescription.java

@@ -19,6 +19,9 @@ import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
+/**
+ * Serialized version of {@link com.velocitypowered.api.plugin.PluginDescription}.
+ */
 public final class SerializedPluginDescription {
 
   public static final Pattern ID_PATTERN = Pattern.compile("[a-z][a-z0-9-_]{0,63}");
@@ -130,6 +133,9 @@ public final class SerializedPluginDescription {
         + '}';
   }
 
+  /**
+   * Represents a dependency.
+   */
   public static final class Dependency {
 
     private final String id;