Revert "Revert scheduler update (#46659)" (#46671)

Reland the new scheduler

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -78,8 +78,8 @@ fn test_remote(cx: &mut App) {
     let guest_buffer = cx.new(|cx| {
         let state = host_buffer.read(cx).to_proto(cx);
         let ops = cx
-            .background_executor()
-            .block(host_buffer.read(cx).serialize_ops(None, cx));
+            .foreground_executor()
+            .block_on(host_buffer.read(cx).serialize_ops(None, cx));
         let mut buffer =
             Buffer::from_proto(ReplicaId::REMOTE_SERVER, Capability::ReadWrite, state, None)
                 .unwrap();