Use num_workers instead of alternate parameter names

[lagru]

Description

Closes #4876. Refer to the closed issue for the original discussion, which settled on workers or num_workers as the consistent name to use. I opted for the latter here, as it seemed more explicit.

Checklist

• A descriptive but concise pull request title
• Docstrings for all functions
• Unit tests
• A gallery example in ./doc/examples for new features
• Contribution guide is followed

Release note

Summarize the introduced changes in the code block below in one or a few sentences. The
summary will be included in the next release notes automatically:

Deprecate parameter `num_threads` in `skimage.restoration.rolling_ball`; 
use `num_workers` instead.

[stefanv]

I will comment on the original issue, but I'm not sure we landed on the correct naming.

[jarrodmillman]

Maybe this should be coordinated across packages via a SPEC.

[lagru]

Marking this as a draft for now.