cx.background_executor().spawn(...) -> cx.background_spawn(...) (#25103)

Done automatically with

> ast-grep -p '$A.background_executor().spawn($B)' -r
'$A.background_spawn($B)' --update-all --globs "\!crates/gpui"

Followed by:

* `cargo fmt`
* Unexpected need to remove some trailing whitespace.
* Manually adding imports of `gpui::{AppContext as _}` which provides
`background_spawn`
* Added `AppContext as _` to existing use of `AppContext`

Release Notes:

- N/A

crates/livekit_client/examples/test_app.rs

@@ -302,11 +302,10 @@ impl LivekitWindow {
         if let Some(track) = self.screen_share_track.take() {
             self.screen_share_stream.take();
             let participant = self.room.local_participant();
-            cx.background_executor()
-                .spawn(async move {
-                    participant.unpublish_track(&track.sid()).await.unwrap();
-                })
-                .detach();
+            cx.background_spawn(async move {
+                participant.unpublish_track(&track.sid()).await.unwrap();
+            })
+            .detach();
             cx.notify();
         } else {
             let participant = self.room.local_participant();