docs: Raise the minimum Sphinx requirement to 2.4.4

Commit 31abfdda (docs: Deprecate use of Sphinx < 2.4.x) in 6.2 added a
warning that support for older versions of Sphinx would be going away.
There have been no complaints, so the time has come.  Raise the minimum
Sphinx version to 2.4.4 and clean out some compatibility code that we no
longer need.

Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/874jgs47fq.fsf@meer.lwn.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>