Move to Qt6 from Qt5

[Tarek-Hasan]

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Your idea.

OnlyOffice Desktop still relies on almost obsolete Qt5. Qt5 is set for EOL at 26 May 2025. Qt5 has few known problems,

• Performance: Qt5 is slower than Qt6. Observing KDE apps ported from Qt5 to 6 the performance jump is huge.
• HiDPI and HDR: Qt5 lacks proper HiDPI & HDR support compared to Qt6. Example HiDPi monitor scale problem Qt/Linux #1771, Incorrect rendering of colors on 10-bit displays on Linux #786
• Security Issues: Maintaining patches for this ageing component takes much time, efforts and resources. Example Outdated and vulnerable v8 is used #1664
• Wayland (Linux specific): Qt lacks full lib-wayland support. Example Blurry appearance on Wayland - switch from XWayland to native Wayland #561
  For a small amount of win7 users, these sacrifices aren't worth making. As OnlyOffice's enterprise consumers use Docs and DocSpace which in my understanding doesn't need Qt.