Update Qt to 6.6

[AlttiRi]

Qt 6.6 have fixed the bug that each call of taking file's mtime performs slow OS' call. That results in very slow sorting by time.
So, opening of an image in a folder with thousands of images will faster hundreds times.

I wrote about it here: #290 (comment)

More info here:
https://forum.qt.io/topic/133954/either-qfileinfo-caching-does-not-work-or-just-its-methods-are-unreasonable-very-slow
https://bugreports.qt.io/browse/QTBUG-100349

https://www.qt.io/blog/qt-6.6-released

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You only need to use QTimeZone::UTC in a sorting function.

Quite old but in qt6.6 QDateTime::lastModified() gained a new parameter 'QTimeZone' where you can pass QTimeZone::UTC to avoid conversion to the local time.
See also https://codereview.qt-project.org/c/qt/qtbase/+/437009

[AlttiRi]

For example, opening an image in a folder with 160k images takes 13 seconds currently.