Motivated by qt5.qtwebengine being marked vulnerable (#435067):
- Move package from pkgs/data/documentation to pkgs/by-name/ze/zeal
- Switch from Qt5 to Qt6 as the default
- Remove zeal-qt5 and zeal-qt6 variants in favor of single Qt6 version
- Add aliases for deprecated Qt-specific variants
(cherry picked from commit 0e805d89c1)
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