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/collab_ui/src/notification_panel.rs

@@ -183,8 +183,7 @@ impl NotificationPanel {
     ) -> Task<Result<Entity<Self>>> {
         cx.spawn(|mut cx| async move {
             let serialized_panel = if let Some(panel) = cx
-                .background_executor()
-                .spawn(async move { KEY_VALUE_STORE.read_kvp(NOTIFICATION_PANEL_KEY) })
+                .background_spawn(async move { KEY_VALUE_STORE.read_kvp(NOTIFICATION_PANEL_KEY) })
                 .await
                 .log_err()
                 .flatten()
@@ -209,7 +208,7 @@ impl NotificationPanel {
 
     fn serialize(&mut self, cx: &mut Context<Self>) {
         let width = self.width;
-        self.pending_serialization = cx.background_executor().spawn(
+        self.pending_serialization = cx.background_spawn(
             async move {
                 KEY_VALUE_STORE
                     .write_kvp(