Qt6: Update to 6.7.1

As a patch release, Qt 6.7.1 does not introduce any new features but
contains more than 400 bug fixes, security updates, and other
improvements to the top of the Qt 6.7.0 release. See more information
about the most important changes and bug fixes from Qt 6.7.1 release
note.

Enable building of databases/qt6-base_sqldriver@mysql with MariaDB. [1]

Fix build of www/qt6-webengine with the SNDIO option enabled. [2]

Announcement: https://www.qt.io/blog/qt-6.7.1-released
Release note: https://code.qt.io/cgit/qt/qtreleasenotes.git/about/qt/6.7.1/release-note.md

PR: 277922 [1], 279180 [2]
Reported by: vvd [1], Benjamin Takacs <nimaje+fbz@bureaucracy.de> [2]
MFH: 2024Q2
Security: f5fa174d-19de-11ef-83d8-4ccc6adda413

(cherry picked from commit 359c8eed1862a08dbe0a9a67a62bb209c9471da9)