Fix voice error 4020 (#1248)

Add type hint to 'd', preventing this from being sent as a string which is now rejected.

src/Discord/Voice/VoiceClient.php

@@ -553,7 +553,7 @@ public function handleWebSocketConnection(WebSocket $ws): void
                     $sendHeartbeat = function () {
                         $this->send([
                             'op' => Op::VOICE_HEARTBEAT,
-                            'd' => microtime(true),
+                            'd' => (int) microtime(true),
                         ]);
                         $this->logger->debug('sending heartbeat');
                         $this->emit('ws-heartbeat', []);