Remove special mapping of auto to {} in open_zarr

[jsignell]

• Closes Inconsistent chunking between xr.open_zarr and xr.open_dataset(..., engine='zarr') with chunks="auto" #11002
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

This PR makes the handling of chunks="auto" consistent between open_zarr and open_dataset(..., engine="zarr").

The handling of chunks still differs in open_zarr vs open_dataset(..., engine="zarr") in that the default in open_zarr is to use chunks={} and a chunk manager (aka dask) when available in your env. And in open_dataset the default is to use chunks=None (aka no chunks).

[dcherian]

let's issue a DeprecationWarning saying the default will switch to chunks=None to match open_dataset. If they want the current behaviour with dask et al, users should pass in chunks={}