www/qt6-webengine: Move PORTREVISION comment

Move inline PORTREVISION comment overhead to prevent editing by overly
enthusiastic custom scripts and to prevent errors from standard tools
like Tools/scripts/bump-revison.sh. We'll have qt7 eventually, but it
will likely be a few years off.

Fixes: 3068854dd026 - libxml2: chase libxml soversion bump
(cherry picked from commit da8c9786240b797f498a35ebff43176e630a2fd5)