Zarr V3 support: use `dimension_names` array metadata rather than user attribute `_ARRAY_DIMENSIONS`

[rabernat]

What is your issue?

Zarr V3 now explicitly supports dimension names on arrays (see Array API docs.

This makes our _ARRAY_DIMENSIONS special attribute unnecessary.

xarray/xarray/backends/zarr.py

Lines 54 to 55 in b133fdc

	# need some special secret attributes to tell us the dimensions
	DIMENSION_KEY = "_ARRAY_DIMENSIONS"

However, we are still using _ARRAY_DIMENSIONS with V3 in Xarray.

It would be great to fix this before we release V3 support so we don't start putting V3 data out into wild with this attribute set.