Some major dependency change in Fcitx 5 for the next release

[Weng Xuetian]

Hi all,
In the next release, C++ standard will be bumped to C++20, the
targeting system will be updated from Ubuntu 20.04 to Ubuntu 24.04,
along with Ubuntu 20.04 EOL.

At the same time, the Qt 5 support in related GUI plugins will also be
dropped where necessary

Thanks,
Xuetian.