Fix QtConcurrent use for future Qt version

Qt 6 has a breaking ABI change in relation to QtConcurrent::run()
https://doc.qt.io/qt-6/concurrent-changes-qt6.html

downloadextractthread.cpp

@@ -171,7 +171,11 @@ void DownloadExtractThread::extractImageRun()
                 }
             }
 
+#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
+            _writeFuture = QtConcurrent::run(&DownloadThread::_writeFile, static_cast<DownloadThread *>(this), _abuf[_activeBuf], size);
+#else
             _writeFuture = QtConcurrent::run(static_cast<DownloadThread *>(this), &DownloadThread::_writeFile, _abuf[_activeBuf], size);
+#endif
             _activeBuf = _activeBuf ? 0 : 1;
             _writeThreadStarted = true;
         }

downloadthread.cpp

@@ -526,7 +526,11 @@ size_t DownloadThread::_writeFile(const char *buf, size_t len)
 
         return _file.seek(len) ? len : 0;
     }
+#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
+    QFuture<void> wh = QtConcurrent::run(&DownloadThread::_hashData, this, buf, len);
+#else
     QFuture<void> wh = QtConcurrent::run(this, &DownloadThread::_hashData, buf, len);
+#endif
 
     qint64 written = _file.write(buf, len);
     _bytesWritten += written;