scripts: sphinx-pre-install: better handle RHEL-based distros

scripts: sphinx-pre-install: better handle RHEL-based distros

Better implement support for RHEL-based distros. While here,
get rid of a Fedora 28 support which cause troubles with
server distros. Also, get rid of yum, as RHEL8 already
suppords dnf, and this is not the minimal version we may
still support.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/4d1b27d3a381f011e150bb50176babba83af9e1a.1754992972.git.mchehab+huawei@kernel.org