Deprecate worker callbacks

We have an older `Client.register_worker_callbacks` system for allowing users to specify a setup function to be run when workers are started. However this system was replaced with the more generic worker plugin system in https://github.com/dask/distributed/pull/2453. Today, `Client.register_worker_callbacks` is actually implemented using our worker plugin system:

https://github.com/dask/distributed/blob/d0a93e950d70680c41959c517a68ee6fc0ce2138/distributed/client.py#L4565-L4583

Deprecating worker callbacks was part of the original plan when the worker plugin system was introduced, I think we just never got around to it. Opening an issue to track deprecating `Client.register_worker_callbacks` in favor of `Client.register_worker_plugin`