Remove EL8-only package because procedure runs on Foreman platform which is EL9 only

[maximiliankolb]

To my understanding, Foreman Server and Smart Proxy Servers are EL9 only for nightly. That means that we can/should drop the python3-lxml package:

$ rg -B 6 "python3-lxml is only needed on EL8 because"
guides/common/modules/proc_registering-project-as-a-client-of-keycloak.adoc
23-. Install the packages required for registering a {keycloak} client:
24-+
25-[options="nowrap", subs="verbatim,quotes,attributes"]
26-----
27-# {project-package-install} mod_auth_openidc keycloak-httpd-client-install python3-lxml
28-----
29:// python3-lxml is only needed on EL8 because of https://issues.redhat.com/browse/RHEL-31496

[ekohl]

Sadly the same bug also affects EL9. I'll update the bug to also mention EL9 and point to https://gitlab.com/redhat/centos-stream/rpms/keycloak-httpd-client-install/-/merge_requests/8.