Qt6: Update to 6.6.3

As a patch release, Qt 6.6.3 does not introduce new features but contains
more than 300 bug fixes, security updates, and other improvements on top
of Qt 6.6.2 release. See more information about the most important
changes and bug fixes in the Qt 6.6.3 release note.

Announcement: https://www.qt.io/blog/qt-6.6.3-released
Release note: https://code.qt.io/cgit/qt/qtreleasenotes.git/about/qt/6.6.3/release-note.md