Begin the process of deprecating the old "async" implementation. This implementation has numerous problems:

• It is fundamentally incompatible with how async executors are expected to work. Trying to combine with other libraries that expect async will either result in a panic, or even undefined behavior.
• They are only usable with a limited set of scheduling primitives, notably Zephyr's k_sem, and k_queue. This is largely a limitation of what is supported by triggered in Zephyr.
• They're not useful for much else. In order to work with something like a driver callback, that interface would have to use a Semaphore or Channel, since the Waker from the async call itself cannot be used.
• They aren't very efficient. Using triggered work ends up using the combined overhead of threads, and work queues. Using the utilities provided by Embassy allows same-thread scheduling to be very low overhead, while bringing in the thread scheduling overhead only when wakes will cross thread boundaries.

The proposed alternative is to use the Executor provided by the feature executor-zephyr. This is a very lightweight executor, making use of some utilities provided by Embassy to allow lightweight scheduling on a single thread, and reasonably efficient scheduling across Zephyr threads. Async device operations can use the Waker mechanism as designed to be able to provide nice async Rust interfaces to drivers.