[Predef-contribute] Language Standards page missing 2020+

[Jim O'H. <joh...@gm...>]

Hello,

C and C++ have updated, thus the values of __STDC_VERSION__ and __cplusplus
have changed on the Language Standards page.

C23 is 202311
C++20 is 202002, C++23 is 202303.

Thanks for keeping this list available.

-Jim