When trying to compile with c++20 support (added in #24169) on macOS 12.3 on master I get inundated with the below warning.

```
 ./configure --enable-c++20 --enable-suppress-external-warnings
```

```
In file included from bitcoin-cli.cpp:19:
In file included from ./util/system.h:20:
./fs.h:71:29: warning: 'u8path<std::string>' is deprecated [-Wdeprecated-declarations]
    return std::filesystem::u8path(utf8_str);
                            ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/filesystem:1570:27: note: 'u8path<std::string>' has been explicitly marked deprecated here
_LIBCPP_INLINE_VISIBILITY _LIBCPP_DEPRECATED_WITH_CHAR8_T
                          ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__config:1078:43: note: expanded from macro '_LIBCPP_DEPRECATED_WITH_CHAR8_T'
#  define _LIBCPP_DEPRECATED_WITH_CHAR8_T _LIBCPP_DEPRECATED
                                          ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__config:1043:48: note: expanded from macro '_LIBCPP_DEPRECATED'
#    define _LIBCPP_DEPRECATED __attribute__ ((deprecated))
                                               ^
1 warning generated.
```

{
    
    
    
    
    
    
    
    
    
    
#ifdef WIN32
    return path.u8string();
#else
    static_assert(std::is_same<path::string_type, std::string>::value, "PathToString not implemented on this platform");
    return path.std::filesystem::path::string();
#endif
}
/**
 * Convert byte string to path object. Inverse of \ref PathToString.
 */
static inline path PathFromString(const std::string& string)