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

stable inclusion
from stable-v5.10.121
commit 0ac587c61fc1ddf536cdbe1c239bc536847d5505
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5L6CQ

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0ac587c61fc1ddf536cdbe1c239bc536847d5505

--------------------------------

commit 627f01ea upstream.

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 <akiyks@gmail.com>
Link: https://lore.kernel.org/r/bd0c2ddc-2401-03cb-4526-79ca664e1cbe@gmail.com


Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>