Fix builds with C++20 (#38501)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38501

char8_t is a distinct type in C++20, which would need to be propagated
everywhere else (e.g. using `std::u8string` instead of `std::string`).
The code was already assuming that char is UTF-8, so we can just cast
accordingly (which works on all compilers: https://godbolt.org/z/9cv4c48o4).

Reviewed By: javache

Differential Revision: D47537998

fbshipit-source-id: ba045483361463f1754e02791114b78f51932a56