Add a section to the standard cpp for minimum support level being c++20.

https://bemanproject.org/docs/beman_standard#c

See this discussion: https://discourse.bemanproject.org/t/potential-policy-change-making-c-20-the-minimum-library-implementation/581/5

This should be something like:

Recommendation: New libraries targeting c++29 should use a minimum of C++20 for implementation. This allows avoiding C++17 patterns such as enable_if and improves the standard facilities available to implementers. However, library authors that are so motivated are allowed to support C++17.