docs: requirements.txt has stopped working again

[Akira Yokosawa <akiyks@xxxxxxxxx>]

Hi all,

Annoyingly, installing Sphinx 2.4.4 using requirements.txt of v6.8-rc1
ends up in a run-time error which looks similar to the one Vegard
reported in commit f4cac0f74658 ("Documentation: constrain alabaster
package to older versions").

The new error is from sphinxcontrib.applehelp which reads:

    Sphinx version error:
    The sphinxcontrib.applehelp extension used by this project needs
    at least Sphinx v5.0; it therefore cannot be built with this version.

Do we want to continue whack-a-mole update for Sphinx 2.4.4?

        Thanks, Akira