cmake: Require Qt 6 to build GUI

CMakeLists.txt

@@ -174,7 +174,7 @@ if(BUILD_GUI)
   if(BUILD_GUI_TESTS)
     list(APPEND qt_components Test)
   endif()
-  find_package(Qt 5.11.3 MODULE REQUIRED
+  find_package(Qt 6.2 MODULE REQUIRED
     COMPONENTS ${qt_components}
   )
   unset(qt_components)