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/semantic_index.rs

@@ -42,8 +42,7 @@ impl SemanticDb {
         cx: &mut AsyncApp,
     ) -> Result<Self> {
         let db_connection = cx
-            .background_executor()
-            .spawn(async move {
+            .background_spawn(async move {
                 std::fs::create_dir_all(&db_path)?;
                 unsafe {
                     heed::EnvOpenOptions::new()
@@ -432,8 +431,7 @@ mod tests {
                 let worktree = search_result.worktree.read(cx);
                 let entry_abs_path = worktree.abs_path().join(&search_result.path);
                 let fs = project.read(cx).fs().clone();
-                cx.background_executor()
-                    .spawn(async move { fs.load(&entry_abs_path).await.unwrap() })
+                cx.background_spawn(async move { fs.load(&entry_abs_path).await.unwrap() })
             })
             .await;