docs/conf.py: Cope with removal of language=None in Sphinx 5.0.0

One of the changes in Sphinx 5.0.0 [1] says [sic]:

5.0.0 final

- #10474: language does not accept None as it value.
The default value of language becomes to 'en' now.

[1]: https://www.sphinx-doc.org/en/master/changes.html#release-5-0-0-released-may-30-2022

It results in a new warning from Sphinx 5.0.0 [sic]:

WARNING: Invalid configuration value found: 'language = None'.
Update your configuration to a valid langauge code. Falling
back to 'en' (English).

Silence the warning by using 'en'.
It works with all the Sphinx versions required for building
kernel documentation (1.7.9 or later).

Signed-off-by: Akira Yokosawa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Cc: [e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/conf.py

···
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = 'en'
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used: