Merge bitcoin/bitcoin#29040: refactor: Remove pre-C++20 code, fs::path cleanup

6666713041 refactor: Rename fs::path::u8string() to fs::path::utf8string() (MarcoFalke)
856c88776f ArgsManager: return path by value from GetBlocksDirPath() (Vasil Dimov)
fa3d9304e8 refactor: Remove pre-C++20 fs code (MarcoFalke)
fa00098e1a Add tests for C++20 std::u8string (MarcoFalke)
fa2bac08c2 refactor: Avoid copy/move in fs.h (MarcoFalke)
faea30227b refactor: Use C++20 std::chrono::days (MarcoFalke)

Pull request description:

  This:

  * Removes dead code.
  * Avoids unused copies in some places.
  * Adds copies in other places for safety.

ACKs for top commit:
  achow101:
    ACK 6666713041
  ryanofsky:
    Code review ACK 6666713041. Just documentation change since last review.
  stickies-v:
    re-ACK 6666713041

Tree-SHA512: 6176e44f30b310d51632ec2d3827c3819905d0ddc6a4b57acfcb6cfa1f9735176da75ee8ed4a4abd1296cb0b83bee9374cc6f91ffac87c19b63c435eeadf3f46

src/qt/guiutil.cpp

@@ -669,7 +669,7 @@ fs::path QStringToPath(const QString &path)
 
 QString PathToQString(const fs::path &path)
 {
-    return QString::fromStdString(path.u8string());
+    return QString::fromStdString(path.utf8string());
 }
 
 QString NetworkToQString(Network net)
@@ -723,8 +723,7 @@ QString ConnectionTypeToQString(ConnectionType conn_type, bool prepend_direction
 
 QString formatDurationStr(std::chrono::seconds dur)
 {
-    using days = std::chrono::duration<int, std::ratio<86400>>; // can remove this line after C++20
-    const auto d{std::chrono::duration_cast<days>(dur)};
+    const auto d{std::chrono::duration_cast<std::chrono::days>(dur)};
     const auto h{std::chrono::duration_cast<std::chrono::hours>(dur - d)};
     const auto m{std::chrono::duration_cast<std::chrono::minutes>(dur - d - h)};
     const auto s{std::chrono::duration_cast<std::chrono::seconds>(dur - d - h - m)};