replace qrand/qsrand with QRandomGenerator (qt6)

Since Qt5, qrand/qsrand methods [are deprecated](https://doc.qt.io/qt-5/qtglobal-obsolete.html#qrand).

Qt6 remove those methods, thus we need to use instead a [QRandomGenerator](https://doc.qt.io/qt-6/qrandomgenerator.html).

QElapsedTimer is available since Qt5.10, and is already used in the source code.

simplecrypt.cpp

@@ -31,6 +31,10 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <QDateTime>
 #include <QCryptographicHash>
 #include <QDataStream>
+#include <QIODevice>
+#if QT_VERSION >= QT_VERSION_CHECK( 5, 10, 0 )
+#include <QtCore/QRandomGenerator>
+#endif
 
 SimpleCrypt::SimpleCrypt():
     m_key(0),
@@ -38,7 +42,9 @@ SimpleCrypt::SimpleCrypt():
     m_protectionMode(ProtectionChecksum),
     m_lastError(ErrorNoError)
 {
+#if QT_VERSION < QT_VERSION_CHECK( 5, 10, 0 )
     qsrand(uint(QDateTime::currentMSecsSinceEpoch() & 0xFFFF));
+#endif
 }
 
 SimpleCrypt::SimpleCrypt(quint64 key):
@@ -47,7 +53,9 @@ SimpleCrypt::SimpleCrypt(quint64 key):
     m_protectionMode(ProtectionChecksum),
     m_lastError(ErrorNoError)
 {
+#if QT_VERSION < QT_VERSION_CHECK( 5, 10, 0 )
     qsrand(uint(QDateTime::currentMSecsSinceEpoch() & 0xFFFF));
+#endif
     splitKey();
 }
 
@@ -113,7 +121,11 @@ QByteArray SimpleCrypt::encryptToByteArray(QByteArray plaintext)
     }
 
     //prepend a random char to the string
+#if QT_VERSION < QT_VERSION_CHECK( 5, 10, 0 )
     char randomChar = char(qrand() & 0xFF);
+#else
+    char randomChar = char(QRandomGenerator::global()->bounded(256));
+#endif
     ba = randomChar + integrityProtection + ba;
 
     int pos(0);