Fix UTC datetime

Author: andreas-el

src/ert/gui/main_window.py

@@ -147,7 +147,9 @@ def slot_add_widget(self, run_dialog: RunDialog) -> None:
             widget.setVisible(False)
 
         run_dialog.setParent(self)
-        date_time = datetime.datetime.now(datetime.UTC).strftime("%Y-%d-%m %H:%M:%S")
+        date_time = datetime.datetime.now(datetime.timezone.utc).strftime(
+            "%Y-%d-%m %H:%M:%S"
+        )
         self.central_panels_map[date_time] = run_dialog
         self.run_dialog_counter += 1
         self.central_layout.addWidget(run_dialog)