US019,US020 02.1 Update ISO 9899 document reference from C11 to C17

We normatively refer to ISO/IEC 9899:2011 which is not the latest version of the C Standard.

Proposed change:
We should be tracking against ISO/IEC 9899:2018 (aka, C17) instead. C17 is near-identical to C11 in that it was a bugfix release that only resolved DRs, with one exception (ATOMIC_VAR_INIT was deprecated in C17)