net-im/telegram-desktop: revive Qt5/Qt6 flavors (+)

• While building with Qt6, Telegram Desktop can use bundled kcoreutils and kimageformat libraries to provide a needed function from KDE Framework
• Switch from bundled dispatch library to devel/libdispatch
• Drop extra -DNDEBUG from CXXFLAGS, it is now a default for ports cmake framework

Revieved by: osa