docs: Deprecate use of Sphinx < 2.4.x

The Sphinx 2.4 release is three years old, and it is becoming increasingly
difficult to even find a system with an sufficiently archaic Python
installation that can run versions older than that.  I can no longer test
changes against anything prior to 2.4.x.

Move toward raising our minimum Sphinx requirement to 2.4.x so we can
delete some older support code and claim to support a range of versions
that we can actually test.

In the absence of screams, the actual removal of support can happen later
in 2023.

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