Replace `mpark.variant` with `std::variant`

[ZedThree]

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[dschwoerer]

It seems std::variant is broken with old gcc versions?

I guess that we should delay this move for now?

It might also be good to check with intel compilers by the time we switch ...

[ZedThree]

Bah, there is a C++17DR for making std::visit work when inheriting from std::variant. The fix is in gcc 11 and clang 15. This works fine in Intel.

So options are:

1. hold off on this for now, which is pretty safe. I think mpark::variant even falls back to std::variant for C++17
2. bump minimum compiler version required to gcc 11. This is mostly safe, most OSes with earlier versions are either EOL or close to EOL.