backwards incompatible changes in reductions

[dcherian]

What is your issue?

I merged #5950 but forgot that it included some backward-incompatible changes (Sorry! this came up in #6403 thanks to @mathause for spotting.)

1. Arguments like keep_attrs, axis are now keyword-only.
2. Some reductions had the 3rd position arg as keep_attrs and in other cases it was axis.

These have been standardized now, and only dim is accepted without kwarg-name.

@pydata/xarray Should we add a deprecation cycle (#5531)? Point (2) above will make it a little messy.

At the very least we should add a deprecation notice before releasing.

[shoyer]

I think this is probably fine without a deprecation cycle. This is a very easy fix for users.

[max-sixty]

@shoyer do you think we need a deprecation cycle for those in #6403? It would be less work if we didn't.

I was thinking that for some of them we did need a depr. cycle, but only because I've seen many cases where specific methods don't use kwargs — e.g. .diff(dim, 2). WDYT?