Merge bitcoin#25808: fs: work around u8path deprecated-declaration warnings with libc++

ced00f5 fs: work around u8path deprecated-declaration warnings with libc++ (fanquake)

Pull request description:

  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>`](https://en.cppreference.com/w/cpp/filesystem/path/u8path) is deprecated starting with C++20.

  Fixes: bitcoin#24682.

ACKs for top commit:
  MarcoFalke:
    review ACK ced00f5
  hebasto:
    ACK ced00f5

Tree-SHA512: f012c4f0bec691090eb3ff128ee0cdc392f73e7857b97131da924ab18c088a82d2fba95316d405feb8b744cba63bfeff7b08143086c173fddbf972139ea0ac0b

Author: MacroFake

build_msvc/common.init.vcxproj.in

@@ -90,7 +90,7 @@
       <AdditionalOptions>/utf-8 /Zc:__cplusplus /std:c++20 %(AdditionalOptions)</AdditionalOptions>
       <DisableSpecificWarnings>4018;4244;4267;4334;4715;4805;4834</DisableSpecificWarnings>
       <TreatWarningAsError>true</TreatWarningAsError>
-      <PreprocessorDefinitions>_SILENCE_CXX20_U8PATH_DEPRECATION_WARNING;_SILENCE_CXX17_CODECVT_HEADER_DEPRECATION_WARNING;ZMQ_STATIC;NOMINMAX;WIN32;HAVE_CONFIG_H;_CRT_SECURE_NO_WARNINGS;_SCL_SECURE_NO_WARNINGS;_CONSOLE;_WIN32_WINNT=0x0601;_WIN32_IE=0x0501;WIN32_LEAN_AND_MEAN;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>_SILENCE_CXX17_CODECVT_HEADER_DEPRECATION_WARNING;ZMQ_STATIC;NOMINMAX;WIN32;HAVE_CONFIG_H;_CRT_SECURE_NO_WARNINGS;_SCL_SECURE_NO_WARNINGS;_CONSOLE;_WIN32_WINNT=0x0601;_WIN32_IE=0x0501;WIN32_LEAN_AND_MEAN;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <AdditionalIncludeDirectories>..\..\src;..\..\src\minisketch\include;..\..\src\univalue\include;..\..\src\secp256k1\include;..\..\src\leveldb\include;..\..\src\leveldb\helpers\memenv;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
     </ClCompile>
     <Link>

src/fs.h

@@ -69,7 +69,11 @@ class path : public std::filesystem::path
 
 static inline path u8path(const std::string& utf8_str)
 {
+#if __cplusplus < 202002L
     return std::filesystem::u8path(utf8_str);
+#else
+    return std::filesystem::path(std::u8string{utf8_str.begin(), utf8_str.end()});
+#endif
 }
 
 // Disallow implicit std::string conversion for absolute to avoid