Collection of breaking changes in Datatree occurring during the migration from xarray-contrib/datatree into pydata/xarray proper

[flamingbear]

What is your issue?

This is an Issue to keep track of changes that have been made to DataTree from the initial migration

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[flamingbear]

Wasn't positive if I wanted to have a block in the actual top level issue or let people just add comments for the breaking changes.

[flamingbear]

- ``Datatree``'s ``as_array`` renamed ``to_dataarray`` to align with ``Dataset``. (:pull:`8789`)
  By `Owen Littlejohns <https://github.com/owenlittlejohns>`_ and `Matt Savoie
  <https://github.com/flamingbear>`_