This fixes the following two warnings detected by recent Sphinx.
1.
Invalid configuration value found: 'language = None'. Update your
configuration to a valid language code. Falling back to 'en' (English).
2.
WARNING: The pre-Sphinx 1.0 'intersphinx_mapping' format is deprecated
and will be removed in Sphinx 8. Update to the current format as
described in the documentation.
These warnings are treated as errors and `tox -e docs` fails.
Change-Id: I6bde5e865d6d750f878fe618ad8e947fc833d899
943a2f474c