docs: change default Qt version from 5 to 6

Update preference from Qt5 to Qt6 in comments.

cMake/FreeCAD_Helpers/ChooseQtVersion.cmake

@@ -41,7 +41,7 @@
 
 # If it is set to Auto(default), FreeCAD finds and uses the
 # version installed on the system. If both versions are
-# found, Qt5 is preferred.
+# found, Qt6 is preferred.
 
 # The output variable is FREECAD_QT_MAJOR_VERSION, which will be either 5 or 6
 
@@ -98,4 +98,4 @@ macro(ChooseQtVersion)
   set(FREECAD_QT_MAJOR_VERSION "${_FREECAD_QT_VERSION}" CACHE INTERNAL
     "Major version number for the Qt installation used.")
   message(STATUS  "Compiling with Qt ${FREECAD_QT_MAJOR_VERSION}")
-endmacro()
+endmacro()