When building in c++20 mode using libc++, the following warning is
emitted:
```bash
./fs.h:72:29: warning: 'u8path<std::string>' is deprecated [-Wdeprecated-declarations]
return std::filesystem::u8path(utf8_str);
^
/usr/lib/llvm-14/bin/../include/c++/v1/__filesystem/u8path.h:72:27: note: 'u8path<std::string>' has been explicitly marked deprecated here
_LIBCPP_INLINE_VISIBILITY _LIBCPP_DEPRECATED_WITH_CHAR8_T
^
/usr/lib/llvm-14/bin/../include/c++/v1/__config:1042:43: note: expanded from macro '_LIBCPP_DEPRECATED_WITH_CHAR8_T'
^
/usr/lib/llvm-14/bin/../include/c++/v1/__config:1007:48: note: expanded from macro '_LIBCPP_DEPRECATED'
^
1 warning generated.
```
as u8path<std::string> is deprecated starting with c++20.
Co-authored-by: MacroFake <falke.marco@gmail.com>
Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>
This is a backport of core#25808