dask-expr got merged into dask

[lahwaacz]

What is your issue?

Since v2.0.0, dask-expr is merged into the main dask package: https://github.com/dask/dask-expr/blob/v2.0/dask_expr/__init__.py

xarray still pulls in dask-expr in various environments for CI: https://github.com/search?q=repo%3Apydata%2Fxarray%20dask-expr&type=code

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[kmuehlbauer]

This also affects current doctests on main.