drop qMapLessThanKey in Qt6 builds

TestQgis::testQMapQVariantList() should check that everything is fine
further reading:
* https://stackoverflow.com/questions/17463889/does-qmap-support-custom-comparator-functions
* https://codereview.qt.nokia.com/c/qt/qtbase/+/297563

python/core/auto_generated/qgis.sip.in

@@ -262,6 +262,7 @@ QVariant data types (such as strings, numeric values, dates and times)
 
 
 
+
 QString qgsVsiPrefix( const QString &path );
 
 

src/core/qgis.cpp

@@ -295,10 +295,12 @@ QString Qgis::devVersion()
   return QString::fromUtf8( QGIS_DEV_VERSION );
 }
 
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 template<>
 bool qMapLessThanKey<QVariantList>( const QVariantList &key1, const QVariantList &key2 )
 {
   // qt's built in qMapLessThanKey for QVariantList is broken and does a case-insensitive operation.
   // this breaks QMap< QVariantList, ... >, where key matching incorrectly becomes case-insensitive..!!?!
   return qgsVariantGreaterThan( key1, key2 ) && key1 != key2;
 }
+#endif

src/core/qgis.h

@@ -561,11 +561,14 @@ CORE_EXPORT bool qgsVariantEqual( const QVariant &lhs, const QVariant &rhs );
  */
 CORE_EXPORT bool qgsVariantGreaterThan( const QVariant &lhs, const QVariant &rhs );
 
+
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
+
 /**
  * Compares two QVariantList values and returns whether the first is less than the second.
  */
 template<> CORE_EXPORT bool qMapLessThanKey<QVariantList>( const QVariantList &key1, const QVariantList &key2 ) SIP_SKIP;
-
+#endif
 
 CORE_EXPORT QString qgsVsiPrefix( const QString &path );
 

src/core/qgscoordinatetransformcontext.cpp

@@ -26,6 +26,7 @@ QString crsToKey( const QgsCoordinateReferenceSystem &crs )
   return crs.authid().isEmpty() ? crs.toWkt( QgsCoordinateReferenceSystem::WKT_PREFERRED ) : crs.authid();
 }
 
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 template<>
 bool qMapLessThanKey<QPair<QgsCoordinateReferenceSystem, QgsCoordinateReferenceSystem>>( const QPair<QgsCoordinateReferenceSystem, QgsCoordinateReferenceSystem> &key1,
     const QPair<QgsCoordinateReferenceSystem, QgsCoordinateReferenceSystem> &key2 )
@@ -34,6 +35,7 @@ bool qMapLessThanKey<QPair<QgsCoordinateReferenceSystem, QgsCoordinateReferenceS
   const QPair< QString, QString > key2String = qMakePair( crsToKey( key2.first ), crsToKey( key2.second ) );
   return key1String < key2String;
 }
+#endif
 
 QgsCoordinateTransformContext::QgsCoordinateTransformContext()
   : d( new QgsCoordinateTransformContextPrivate() )