docs: update recommended Sphinx version to 2.4.4

There are some docs that have nested tables. While this was
always part of the spec, only Sphinx version 2.4.x can
translate it to LaTeX.

In other words, if someone is using a Sphinx version < 2.4,
the LaTeX and PDF output won't work for some of the docs.

So, it seems that it is time to raise the bar again
for the recommented version.

The Sphinx check script is already smart enough to keep
working, with older versions, warning the users that
an upgrade is recommended (and explaining how):

	Sphinx version 1.7.9
	Warning: It is recommended at least Sphinx version 2.4.4.
	Detected OS: Fedora release 31 (Thirty One).

	To upgrade Sphinx, use:

		/usr/bin/virtualenv sphinx_2.4.4
		. sphinx_2.4.4/bin/activate
		pip install -r ./Documentation/sphinx/requirements.txt
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/498f701c618f7d0cf5f0a37e5889ee926f7c8bf4.1586881715.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sphinx/requirements.txt

 docutils
-Sphinx==1.7.9
+Sphinx==2.4.4
 sphinx_rtd_theme