Added changes due to comments

application/src/test/java/org/thingsboard/server/transport/mqtt/mqttv3/credentials/BasicMqttCredentialsTest.java

@@ -121,6 +121,7 @@ public class BasicMqttCredentialsTest extends AbstractMqttIntegrationTest {
         MqttTestClient mqttTestClient = new MqttTestClient(CLIENT_ID);
         try {
             mqttTestClient.connectAndWait(USER_NAME3, "WRONG PASSWORD");
+            Assert.fail(); // This should not happens, because we have a wrong password
         } catch (MqttException e) {
             Assert.assertEquals(4, e.getReasonCode()); // 4 - Reason code for bad username or password in MQTT v3
         }

application/src/test/java/org/thingsboard/server/transport/mqtt/mqttv5/MqttV5TestClient.java

@@ -27,7 +27,7 @@ import org.thingsboard.server.common.data.StringUtils;
 
 import java.util.concurrent.TimeUnit;
 
-public class MqttV5TestClient {
+public class MqttV5TestClient { // We should copy part of MqttV3TestClient, due to different package names in import
 
     private static final String MQTT_URL = "tcp://localhost:1883";
     private static final int TIMEOUT = 30; // seconds

common/transport/mqtt/src/main/java/org/thingsboard/server/transport/mqtt/MqttTransportHandler.java

@@ -947,7 +947,6 @@ public class MqttTransportHandler extends ChannelInboundHandlerAdapter implement
             case 5:
                 return MqttVersion.MQTT_5;
             default:
-            case 4:
                 return MqttVersion.MQTT_3_1_1;
         }
     }

common/transport/mqtt/src/main/java/org/thingsboard/server/transport/mqtt/session/DeviceSessionCtx.java

@@ -79,8 +79,6 @@ public class DeviceSessionCtx extends MqttDeviceAwareSessionContext {
     @Setter
     private MqttVersion mqttVersion;
 
-
-
     private volatile MqttTopicFilter telemetryTopicFilter = MqttTopicFilterFactory.getDefaultTelemetryFilter();
     private volatile MqttTopicFilter attributesTopicFilter = MqttTopicFilterFactory.getDefaultAttributesFilter();
     private volatile TransportPayloadType payloadType = TransportPayloadType.JSON;