Conda defaults channel?

[bryevdv]

CI summaries have lots of these warnings as clutter:

The 'defaults' channel might have been added implicitly. If this is intentional, add 'defaults' to the 'channels' list. Otherwise, consider setting 'conda-remove-defaults' to 'true'.Show less

Do we need the defaults channel (I don't actually know)? We should update the config either way.

@bokeh/dev

[mattpap]

There's issue #14244 for tracking this.