Meson: Correct QT_DISABLE_DEPRECATED_BEFORE for Qt6-mode
Currently meson based build sets QT_DISABLE_DEPRECATED_BEFORE to be low enough for Qt5-mode build also. It should depend on qtver so that Qt6-mode build (which is the default!) would get its own value.
Currently meson based build sets QT_DISABLE_DEPRECATED_BEFORE to be low enough for Qt5-mode build also. It should depend on qtver so that Qt6-mode build (which is the default!) would get its own value.