Allow to build against newer qt-6.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -151,7 +151,7 @@ set(GPGME_VERSION "1.19")
 set(LCMS2_REQUIRED_VERSION "2.14")
 set(TIFF_VERSION "4.5")
 set(QT5_VERSION "5.15")
-set(QT6_VERSION "6.4")
+set(QT6_VERSION "6")
 set(BOOST_VERSION "1.83")
 set(GOBJECT_INTROSPECTION_VERSION "1.80")
 
