documentation update: RHEL 9

[donsizemore]

Dataverse is known to install properly on Rocky 9, a derivative of RHEL 9.

The installation instructions target RHEL 7 and make accommodation for RHEL 8. I propose we target RHEL 9 and retain accommodations for RHEL 8, particularly since RHEL 7 EOLs in two years.

Pull request forthcoming.

[donsizemore]

Per discussion with @pdurbin this is on hold pending availability of Shibboleth packages for RHEL 9 at: https://shibboleth.net/downloads/service-provider/RPMS/

[donsizemore]

@pdurbin Shibboleth now offers RPM packages for Rocky 9, but they're not specifically recommended for RHEL 9:

[shibboleth]
name=Shibboleth (rockylinux9)
# Please report any problems to https://shibboleth.atlassian.net/jira
type=rpm-md
mirrorlist=https://shibboleth.net/cgi-bin/mirrorlist.cgi/rockylinux9
gpgcheck=1
gpgkey=https://shibboleth.net/downloads/service-provider/RPMS/repomd.xml.key
        https://shibboleth.net/downloads/service-provider/RPMS/cantor.repomd.xml.key
enabled=1