fmt: Replace deprecated fmt::localtime usage with Common::LocalTime

Source/Core/Common/SettingsHandler.cpp

@@ -122,7 +122,6 @@ std::string SettingsWriter::GenerateSerialNumber()
 
   // Must be 9 characters at most; otherwise the serial number will be rejected by SDK libraries,
   // as there is a check to ensure the string length is strictly lower than 10.
-  // 3 for %j, 2 for %H, 2 for %M, 2 for %S.
-  return fmt::format("{:%j%H%M%S}", fmt::localtime(t));
+  return fmt::format("{:09}", t % 1000000000);
 }
 }  // namespace Common