fix: (cmake) Fix Qt version check variable name

The condition used `Qt6_VERSION`, which is not defined by `find_package(QT ...)`.
Replace it with the correct variable `QT_VERSION` to ensure the version check
works properly with Qt 6.10 and later.

Author: hillwoodroc

CMakeLists.txt

@@ -21,7 +21,7 @@ option(ENABLE_COV "Enable code coverage" OFF)
 if(PROJECT_VERSION_MAJOR EQUAL 6)
   set(VERSION_SUFFIX 6)
   find_package(QT NAMES Qt6 REQUIRED COMPONENTS Core)
-  if(Qt6_VERSION VERSION_GREATER_EQUAL 6.10)
+  if(QT_VERSION VERSION_GREATER_EQUAL 6.10)
     find_package(Qt6 COMPONENTS CorePrivate GuiPrivate WidgetsPrivate REQUIRED)
   endif()
 else()