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/Viewer/SituationMap/SymbologyImpl/base_symbol_model.cpp

@@ -163,7 +163,7 @@ QString BaseSymbolItem::symbolSet()
 {
   if (m_symbolSet.isEmpty())
   {
-    SidcDefinitions::SymbolSet symbolSet = SidcDefinitions::castToSymbolSet(m_symbolProperties.originalKey().left(2).toUInt()); // Qt6: leftRef(2) removed
+    SidcDefinitions::SymbolSet symbolSet = SidcDefinitions::castToSymbolSet(m_symbolProperties.originalKey().left(2).toUInt());
     m_symbolSet = QString(QStringLiteral("%1 - ")).arg(static_cast<int>(symbolSet)) + AdvancedSymbology::SidcDefinitions::toString(symbolSet);
   }
   return m_symbolSet;
@@ -171,7 +171,7 @@ QString BaseSymbolItem::symbolSet()
 
 int BaseSymbolItem::symbolSetAsInt()
 {
-  return static_cast<int>(SidcDefinitions::castToSymbolSet(m_symbolProperties.originalKey().left(2).toUInt())); // Qt6: leftRef(2) removed
+  return static_cast<int>(SidcDefinitions::castToSymbolSet(m_symbolProperties.originalKey().left(2).toUInt()));
 }