Move "async move" a few characters to the left in cx.spawn() (#26758)

This is the core change:
https://github.com/zed-industries/zed/pull/26758/files#diff-044302c0d57147af17e68a0009fee3e8dcdfb4f32c27a915e70cfa80e987f765R1052

TODO:
- [x] Use AsyncFn instead of Fn() -> Future in GPUI spawn methods
- [x] Implement it in the whole app
- [x] Implement it in the debugger 
- [x] Glance at the RPC crate, and see if those box future methods can
be switched over. Answer: It can't directly, as you can't make an
AsyncFn* into a trait object. There's ways around that, but they're all
more complex than just keeping the code as is.
- [ ] Fix platform specific code

Release Notes:

- N/A

crates/git_ui/src/onboarding.rs

@@ -184,9 +184,11 @@ fn get_dismissed() -> bool {
 }
 
 fn persist_dismissed(cx: &mut App) {
-    cx.spawn(|_| {
+    cx.spawn(async |_| {
         let time = chrono::Utc::now().to_rfc3339();
-        db::kvp::KEY_VALUE_STORE.write_kvp(DISMISSED_AT_KEY.into(), time)
+        db::kvp::KEY_VALUE_STORE
+            .write_kvp(DISMISSED_AT_KEY.into(), time)
+            .await
     })
     .detach_and_log_err(cx);
 }
@@ -202,8 +204,12 @@ pub(crate) fn clear_dismissed(cx: &mut App) {
             });
     });
 
-    cx.spawn(|_| db::kvp::KEY_VALUE_STORE.delete_kvp(DISMISSED_AT_KEY.into()))
-        .detach_and_log_err(cx);
+    cx.spawn(async |_| {
+        db::kvp::KEY_VALUE_STORE
+            .delete_kvp(DISMISSED_AT_KEY.into())
+            .await
+    })
+    .detach_and_log_err(cx);
 }
 
 impl Render for GitBanner {