Fully deprecate squeeze kwarg to groupby (#9280)

* Fully deprecate squeeze kwarg to groupby

Closes #9279
Closes #1460
Closes #2157

* Fix doctests

* Update xarray/core/groupby.py

Co-authored-by: Maximilian Roos <[email protected]>

* Fix whats-new

* Update doc/whats-new.rst

---------

Co-authored-by: Maximilian Roos <[email protected]>

Author: dcherian

doc/whats-new.rst

@@ -49,10 +49,10 @@ Breaking changes
   ``origin`` or ``offset`` parameters is recommended as a replacement for using
   the ``base`` parameter and using time offset arithmetic is recommended as a
   replacement for using the ``loffset`` parameter.
-
-
-Deprecations
-~~~~~~~~~~~~
+- The ``squeeze`` kwarg to ``groupby`` is completely deprecated. This has been the source of some quite confusing
+  behaviour and has been deprecated since v2024.01.0. `groupby`` behavior is now always consistent
+  with the existing ``.groupby(..., squeeze=False)`` behavior.
+  By `Deepak Cherian <https://github.com/dcherian>`_. (:pull:`9280`)
 
 
 Bug fixes