feat(swarm): downgrade connection limits warning from warn to debug

This tiny PR contains a proposed change for the warning related to the exceeded pending incoming connections limit. This warning can't be controlled and causes user confusion. It contains useful technical info but IMHO should be downgraded from `warn` to `debug`.

Pull-Request: #3671.

swarm/src/lib.rs

@@ -1111,7 +1111,7 @@ where
                                 error: &error,
                                 connection_id,
                             }));
-                        log::warn!("Incoming connection rejected: {:?}", connection_limit);
+                        log::debug!("Incoming connection rejected: {:?}", connection_limit);
                     }
                 };
             }