refactor: clean up verbose migration comments

Remove redundant Qt6 migration comments that restate obvious API changes:
- QtConcurrent::run parameter order explanations (preserved where lambda wrapping needed)
- QStringRef/QStringView API changes (shortened to essential info)
- QComboBox::activated signal signature change
- QMap::unite() removal semantics
- QAction include path change
- QMouseEvent/QWheelEvent position() API
- QList/QVector equivalence in Qt6
- QFuture continuation API
- Code generation comments

Preserved comments explaining non-obvious Qt6 behavior or necessary
API migration details.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

qt6-after/qt6-migrated-legatus/Lts/Tools/TextEditorTest/textedit.cpp

@@ -367,7 +367,7 @@ bool TextEdit::load(const QString &f)
     return false;
 
   QByteArray data = file.readAll();
-  // Qt6: QTextCodec/Qt::codecForHtml removed; decode as UTF-8
+  // QTextCodec removed in Qt6; use UTF-8
   QString str = QString::fromUtf8(data);
   if (Qt::mightBeRichText(str)) {
     QUrl baseUrl = (f.front() == QLatin1Char(':') ? QUrl(f) : QUrl::fromLocalFile(f)).adjusted(QUrl::RemoveFilename);