docs(groupby): mention deprecation of squeeze kwarg (#9625)

As mentioned in #2157, the docstring of `Dataset.groupby` does not
reflect deprecation of squeeze (as the docstring of `DataArray.groupby`
does) and states an incorrect default value.

xarray/core/dataset.py

@@ -10376,10 +10376,8 @@ def groupby(
             Array whose unique values should be used to group this array. If a
             Hashable, must be the name of a coordinate contained in this dataarray. If a dictionary,
             must map an existing variable name to a :py:class:`Grouper` instance.
-        squeeze : bool, default: True
-            If "group" is a dimension of any arrays in this dataset, `squeeze`
-            controls whether the subarrays have a dimension of length 1 along
-            that dimension or if the dimension is squeezed out.
+        squeeze : False
+            This argument is deprecated.
         restore_coord_dims : bool, default: False
             If True, also restore the dimension order of multi-dimensional
             coordinates.