meta-qt6: switch to 6.4 branch

Change-Id: Ib4777a5bb1adf9c82ab955a37e1fe8d717ce04ad
Reviewed-by: Pasi Petäjäjärvi <pasi.petajajarvi@qt.io>
(cherry picked from commit 5fc83ec5)
Reviewed-by: Samuli Piippo <samuli.piippo@qt.io>

scripts/manifest.xml

@@ -31,8 +31,8 @@
           fetch="ssh://codereview.qt-project.org/yocto"/>
   <project name="meta-qt6"
            remote="qt"
-           revision="dev"
-           upstream="dev"
+           revision="6.4"
+           upstream="6.4"
            path="sources/meta-qt6"/>
 
   <!-- NXP -->