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/collab_panel.rs

@@ -319,8 +319,7 @@ impl CollabPanel {
         mut cx: AsyncWindowContext,
     ) -> anyhow::Result<Entity<Self>> {
         let serialized_panel = cx
-            .background_executor()
-            .spawn(async move { KEY_VALUE_STORE.read_kvp(COLLABORATION_PANEL_KEY) })
+            .background_spawn(async move { KEY_VALUE_STORE.read_kvp(COLLABORATION_PANEL_KEY) })
             .await
             .map_err(|_| anyhow::anyhow!("Failed to read collaboration panel from key value store"))
             .log_err()
@@ -351,7 +350,7 @@ impl CollabPanel {
     fn serialize(&mut self, cx: &mut Context<Self>) {
         let width = self.width;
         let collapsed_channels = self.collapsed_channels.clone();
-        self.pending_serialization = cx.background_executor().spawn(
+        self.pending_serialization = cx.background_spawn(
             async move {
                 KEY_VALUE_STORE
                     .write_kvp(