fix: Don't generate new timestamp for re-sent messages (#7889)

Timestamp renewal was introduced in 1dbf924c6a "feat:
chat::resend_msgs: Guarantee strictly increasing time in the Date header" so that re-sent messages
can be deduplicated on the reciver side, but the deduplication logic doesn't depend on "Date"
anymore.

src/chat/chat_tests.rs

@@ -2641,7 +2641,7 @@ async fn test_resend_own_message() -> Result<()> {
     );
     let msg_from = Contact::get_by_id(&fiona, msg.get_from_id()).await?;
     assert_eq!(msg_from.get_addr(), "alice@example.org");
-    assert!(sent1_ts_sent < msg.timestamp_sent);
+    assert!(sent1_ts_sent == msg.timestamp_sent);
 
     Ok(())
 }