Require GCC11 for full C++20 support

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.