feat: use `async_executor::StaticExecutor`

[jbr]

The StaticExecutor type introduced in [email protected] seems well suited for the async-global-executor use case, so this PR adopts that optimization.

My initial intent was also to adopt StaticLocalExecutor but I'm unclear on the intended usage of that type[^1] and so it was left for future work.

[^1]: StaticLocalExecutor::spawn requires a 'static thread local lifetime, but LocalKey intentionally avoids surfacing 'static lifetimes.

Thanks!