refactor(ui): AllRemoteEvents::back becomes last to add semantics.

This patch renames `AllRemoteEvents::back` to `last` so that it now gets
a specific semantics instead of being generic.

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

@@ -1529,7 +1529,7 @@ impl TimelineController {
     /// it's folded into another timeline item.
     pub(crate) async fn latest_event_id(&self) -> Option<OwnedEventId> {
         let state = self.state.read().await;
-        state.meta.all_remote_events.back().map(|event_meta| &event_meta.event_id).cloned()
+        state.meta.all_remote_events.last().map(|event_meta| &event_meta.event_id).cloned()
     }
 }
 

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

@@ -1183,7 +1183,7 @@ impl AllRemoteEvents {
     }
 
     /// Return a reference to the last remote event if it exists.
-    pub fn back(&self) -> Option<&EventMeta> {
+    pub fn last(&self) -> Option<&EventMeta> {
         self.0.back()
     }
 }