warn about outdated app

src/stock.rs

@@ -225,6 +225,12 @@ pub enum StockMessage {
                     Adjust your clock ⏰🔧 to ensure your messages are received correctly."
     ))]
     BadTimeMsgBody = 85,
+
+    #[strum(props(fallback = "⚠️ Your Delta Chat version might be outdated.\n\n\
+                    This may cause problems because your chat partners use newer versions - \
+                    and you are missing the latest features 😳\n\
+                    Please check https://get.delta.chat or your app store for updates."))]
+    UpdateReminderMsgBody = 86,
 }
 
 /*