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/semantic_index/src/worktree_index.rs

@@ -52,8 +52,7 @@ impl WorktreeIndex {
         cx.spawn(|mut cx| async move {
             let entries_being_indexed = Arc::new(IndexingEntrySet::new(status_tx));
             let (embedding_index, summary_index) = cx
-                .background_executor()
-                .spawn({
+                .background_spawn({
                     let entries_being_indexed = Arc::clone(&entries_being_indexed);
                     let db_connection = db_connection.clone();
                     async move {