Add rancher-selinux's rpm install guide for EL9

[andypitcher]

This PR implements this issue:

• Update Rancher docs with instructions for EL9 rancher-selinux#28

Description

• This PR appends rancher-selinux rpm install by adding rancher-selinux's rpm install guide for EL9 (setup yum repo)
• Changes (latest, v2.8, v2.7): reference-guides/rancher-security/selinux-rpm/about-rancher-selinux.md

# cat << EOF > /etc/yum.repos.d/rancher.repo
[rancher]
name=Rancher
baseurl=https://rpm.rancher.io/rancher/production/centos/9/noarch
enabled=1
gpgcheck=1
gpgkey=https://rpm.rancher.io/public.key
EOF

[btat]

Hi @andypitcher @macedogm, the reference issue mentions After EL9 packages are officially released.... Has that already happened (i.e. if this PR is approved can it be merged right away)?

[macedogm]

@btat no, that hasn't happened yet and it depends on rancher/rancher#43146. I believe that the best approach here is for us to track this and do the merge when the package is released. WDYT?

[btat]

@btat no, that hasn't happened yet and it depends on rancher/rancher#43146. I believe that the best approach here is for us to track this and do the merge when the package is released. WDYT?

Sounds good to me.

[andypitcher]

@macedogm @btat we should be good to go, the production packages have been released for RHEL9, see comment.
I just have rebased, so ready to review/merge.