Propose deprecations in astropy/coordinates/matrix_utilities.py

[Pey Lian Lim]

From the change log:

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The ``matrix_utilities`` module was not included in the ``astropy`` API
documentation, but it was nonetheless explicitly referred to in some of the
other documentation.
This made it unclear if the functions in the module are public or private.
The public matrix utilities ``is_rotation_or_reflection()`` and
``rotation_matrix()`` have been made available from the ``astropy.coordinates``
namespace and should be imported from there.
Functions not available from the ``astropy.coordinate`` namespace are private
and may be changed or removed without warning.
However, three functions have been explicitly deprecated, despite being
private, as a courtesy to existing users.
``matrix_utilites.angle_axis()`` and ``matrix_utilites.is_rotation()`` are
deprecated without replacement.
``matrix_utilities.is_O3()`` is deprecated and the public
``is_rotation_or_reflection()`` function can be used as a replacement.

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If you use any of these downstream, please comment over at https://github.com/astropy/astropy/pull/18418 which is currently milestoned for v7.2. Thank you!