Use a more C++11 way to compute file's age

C++20 was a bit to much for older compilers (clang-11 and g++-10)

src/language/modules/CoreModule.cpp

@@ -152,14 +152,13 @@ CoreModule::CoreModule() : BuiltinModule(true)
                                 bool has_stop_file = false;
 
                                 if (parallel::rank() == 0) {
-                                  std::filesystem::path stop_file("STOP");
+                                  std::filesystem::path stop_file("stop");
                                   if (std::filesystem::exists(stop_file)) {
                                     const double elapse_time = ExecutionStatManager::getInstance().getElapseTime();
 
                                     const double stop_file_age = std::chrono::duration_cast<std::chrono::seconds>(
-                                                                   std::chrono::system_clock::now() -
-                                                                   std::chrono::clock_cast<std::chrono::system_clock>(
-                                                                     std::filesystem::last_write_time(stop_file)))
+                                                                   std::filesystem::file_time_type::clock::now() -
+                                                                   std::filesystem::last_write_time(stop_file))
                                                                    .count();
 
                                     has_stop_file = elapse_time > stop_file_age;