Draft: Move our build requirements to c++20

This is an exploratory branch to see how CI and the code reacts. I assume we have to also upgrade our CI images at least from clang-8 to clang-10. For gcc we would also need to go to v10.

cmake is required to 3.12 (up from 3.9).

Why?

• better error messages
• better handling of templates, need to check compile time...
• new "concepts" as a proper way to handle template requirements
• lots of new features, see e.g. https://www.geeksforgeeks.org/features-of-c-20/