scripts: sphinx-pre-install: cleanup rhel support

scripts: sphinx-pre-install: cleanup rhel support

Rhel < 8.0 is not supported anymore. Drop support for it.

Rhel 8 is problematic: at least on the tests I did with a
docker repo, it didn't work, but it could be due to the issue
that it is actually different than a real One.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/62fe8ab243ad39f4964f1f74b965e43dc8f10e23.1754992972.git.mchehab+huawei@kernel.org