SPIGOT-6357: Add World#getMinHeight

By: md_5 <git@md-5.net>

paper-server/src/main/java/org/bukkit/craftbukkit/CraftWorld.java

@@ -1856,6 +1856,11 @@ public class CraftWorld implements World {
         return world.getChunkProvider().allowMonsters;
     }
 
+    @Override
+    public int getMinHeight() {
+        return 0;
+    }
+
     @Override
     public int getMaxHeight() {
         return world.getBuildHeight();