ThreadUtils: Replace the implementation for Qt 6.8

Change-Id: I8d4890179165bca02af972debfe4f11fae41ebbd
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/threadutils.cpp

@@ -10,7 +10,12 @@ namespace Utils {
 
 bool isMainThread()
 {
+#if QT_VERSION >= QT_VERSION_CHECK(6, 8, 0)
+    // TODO: Remove threadutils.h and replace all usages with:
+    return QThread::isMainThread();
+#else
     return QThread::currentThread() == qApp->thread();
+#endif
 }
 
 } // namespace Utils