fix：【iot 物联网】增强 jdk8 的兼容性

yudao-module-iot/yudao-module-iot-gateway/src/main/java/cn/iocoder/yudao/module/iot/gateway/protocol/mqtt/manager/IotMqttConnectionManager.java

@@ -125,7 +125,7 @@ public class IotMqttConnectionManager {
      */
     public IotMqttConnectionManager.ConnectionInfo getConnectionInfoByDeviceId(Long deviceId) {
         // 通过设备 ID 获取连接端点
-        var endpoint = getDeviceEndpoint(deviceId);
+        MqttEndpoint endpoint = getDeviceEndpoint(deviceId);
         if (endpoint == null) {
             return null;
         }