Truncate ping response packets (#9446)

Previously all ping response packets were 0-padded to 500 bytes. By
truncating we only send the bytes that are actually needed.

core/src/mindustry/net/ArcNetProvider.java

@@ -94,7 +94,9 @@ public class ArcNetProvider implements NetProvider{
         server.setMulticast(multicastGroup, multicastPort);
         server.setDiscoveryHandler((address, handler) -> {
             ByteBuffer buffer = NetworkIO.writeServerData();
+            int length = buffer.position();
             buffer.position(0);
+            buffer.limit(length);
             handler.respond(buffer);
         });