replace QTime with QElapsedTimer when possible (qt6)

Since Qt5, some methods of QTime [are deprecated](https://doc.qt.io/qt-5/qtime-obsolete.html).

Qt6 remove those methods, thus in two instances we need to use
a [QElapsedTimer](https://doc.qt.io/qt-6/qelapsedtimer.html) instead of a QTime.

QElapsedTimer is available since Qt5.4, and is already used in the source
code.

mainwindow.h

@@ -137,7 +137,7 @@ private:
     DBCHandler *dbcHandler;
     QByteArray inputBuffer;
     QTimer updateTimer;
-    QTime *elapsedTime;
+    QElapsedTimer *elapsedTime;
     int framesPerSec;
     int rxFrames;
     bool inhibitFilterUpdate;