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/repl/src/repl_store.rs

@@ -164,7 +164,7 @@ impl ReplStore {
 
         let remote_kernel_specifications = self.get_remote_kernel_specifications(cx);
 
-        let all_specs = cx.background_executor().spawn(async move {
+        let all_specs = cx.background_spawn(async move {
             let mut all_specs = local_kernel_specifications
                 .await?
                 .into_iter()