Require GCC11 for full C++20 support

[jhale]

There are quite a few C++20 features that are only supported in GCC11 and above, e.g. using enum would be a big one for code readability in basix.

Currently we are using ubuntu-20.04 and GCC10 GitHub runners (i.e. no Docker) for pythonapp.yml: https://github.com/FEniCS/basix/blob/main/.github/workflows/pythonapp.yml

I would propose:

• Moving to running the tests in pythonapp.yml inside a plain Ubuntu Docker container so that the dependancy surface is minimal and well-controlled.
• Switching to GCC11 so that we can use a full C++20 feature set.

[jhale]

I started making some C++20 changes here: https://github.com/FEniCS/basix/tree/jhale/cell-fixes

[jhale]

Other tests are also running directly on GitHub action images. While I guess it reduces test runtime, I think it's better practice to start from a very minimal image and install what's needed (not a lot for Basix, presumably?).

[IgorBaratta]

I like the idea of enabling full C++ 20 support.
But I also think it can make it difficult to build basix on non x86_64 computers.

I think it may conflict with the idea of basix being pip installable, because it would require a compiler not available in most OS.