Use the new tokio `LocalRuntime` in `actix_rt`

[Sytten]

This is a tracking issue

Tokio is deprecating the LocalSet (https://github.com/tokio-rs/tokio/issues/6741) in favour of a new LocalRuntime (https://github.com/tokio-rs/tokio/issues/6739) which will avoid panics if a user mixes tokio::spawn with tokio::spawn_local/actix_rt::spawn. It should also improve performances.

Once this is released we should migrate to the new runtime.