keep_attrs for NamedArray

[dcherian]

What is your issue?

Copying over @max-sixty's comment from #8304 (comment)

I haven't been in touch with the NameArray discussions so forgive a glib comment — but re #3891 — this would be a "once-in-a-library" opportunity to always retain attrs in aggregations, removing the keep_attrs option in methods.

(Xarray could still handle them as it wished, so xarray's external interface wouldn't need to change immediately...)

@pydata/xarray Should we just delete the keep_attrs kwarg completely for NamedArray and always propagate attrs? obj.attrs.clear() seems just as easy to type.