fix(ui): Demote aggregation target not found log to TRACE level

We encountered this warning a lot in the logs after upgrading the SDK today.

My understanding is that this path is expected if the event is not yet in the timeline, so it's nothing to warn about.

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

crates/matrix-sdk-ui/src/timeline/controller/aggregations.rs

@@ -399,7 +399,7 @@ pub(crate) fn find_item_and_apply_aggregation(
     aggregation: Aggregation,
 ) -> bool {
     let Some((idx, event_item)) = rfind_event_by_item_id(items, target) else {
-        warn!("couldn't find aggregation's target {target:?}");
+        trace!("couldn't find aggregation's target {target:?}");
         return false;
     };