fix: chat-description-changed text in old clients (#7870)

instead of Alice saying to Bob "You changed the chat description",
we now say "[Chat description changed, please update ...]

i was also considering to say "[Chat description changed to:\n\n...]"
but then there is no incentive for ppl to update, and chat descriptions
for chat creation would still be missing. and this is probably far more
often used.

successor of https://github.com/chatmail/core/pull/7829

src/chat/chat_tests.rs

@@ -3231,7 +3231,7 @@ async fn test_chat_description(initial_description: &str, join_via_qr: bool) ->
         let sent = alice.pop_sent_msg().await;
         assert_eq!(
             sent.load_from_db().await.text,
-            "You changed the chat description."
+            "[Chat description changed. To see this and other new features, please update the app]"
         );
 
         tcm.section("Bob receives the description change");