CMake: Require C++20 support in compiler

As reported in #96 (closed), building with g++ version 7 will fail, because it lacks support for C++ designated initializers, a feature that was introduced with the C++20 language standard, and supported by GCC starting with version 8.

To make the requirement for this (and possibly other) features more explicit, raise the CXX_STANDARD level to 20, and set CXX_STANDARD_REQUIRED to prevent fallback to earlier standards on compilers without C++20 support.

Also, update the README to mention C++20 (instead of C++17) as a requirement.

Fixes #96 (closed)