chore: Prepare for Rust edition bump to 2024 (without autofix) (#27791)

Successor to #27779 - in this PR I've applied changes manually, without
futzing with if let lifetimes at all.

Release Notes:

- N/A

crates/gpui/src/subscription.rs

@@ -45,7 +45,7 @@ where
         &self,
         emitter_key: EmitterKey,
         callback: Callback,
-    ) -> (Subscription, impl FnOnce()) {
+    ) -> (Subscription, impl FnOnce() + use<EmitterKey, Callback>) {
         let active = Rc::new(Cell::new(false));
         let mut lock = self.0.lock();
         let subscriber_id = post_inc(&mut lock.next_subscriber_id);
@@ -88,7 +88,10 @@ where
         (subscription, move || active.set(true))
     }
 
-    pub fn remove(&self, emitter: &EmitterKey) -> impl IntoIterator<Item = Callback> {
+    pub fn remove(
+        &self,
+        emitter: &EmitterKey,
+    ) -> impl IntoIterator<Item = Callback> + use<EmitterKey, Callback> {
         let subscribers = self.0.lock().subscribers.remove(emitter);
         subscribers
             .unwrap_or_default()