Replace legacy commands with 'dask worker' and 'dask scheduler'.

[wilson]

Hi there. I'm not 100% the change I made re: GPU workers is correct. I believe it to be, but I haven't used it in anger. Looking at the dask-cuda project, it does look like dask-cuda-worker is deprecated just like dask-worker.

Anyway, this PR fixes a pair of warnings:

distributed/cli/dask_scheduler.py:140: FutureWarning: dask-scheduler is deprecated and will be removed in a future release; use `dask scheduler` 
distributed/cli/dask_worker.py:264: FutureWarning: dask-worker is deprecated and will be removed in a future release; use `dask worker` instead

In doing this, I realized that the worker_command option is not used anywhere, so this PR also removes the remaining references to it.

[wilson]

This look great thanks.

In dask-cuda we replaced dask-cuda-worker with dask cuda worker. That's all that should be changed. Sorry you went down the resources path, that's not needed.

Oh, gotcha; I'll change it up.

[wilson]

OK, rebased off main and all looks green.

[jacobtomlinson]

There's been no activity here for a while so I'm going to close this PR out. @wilson if you have the desire to pick this up I'd happily give it another review.