There is a new version of the ISO C standard, so we should update our normative reference, and update the header synopses in the library clauses to match the content of the C standard library. A similar change to rebase C++17 on C11 was previously done via [P0063R3: C++17 should refer to C11 instead of C99](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0063r3.html) (see R0 for more detailed rationale) and then to trivially rebase on C17 via [C++20 ballot comment US-019](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1971r0.html#US019,%20US020) (which just updated the normative reference, as there were no changes affecting the contents of the library in C17).