Add admonition about rkhunter being dead

[sspencerwire]

• admonition added for version 9

Author checklist (Completed by original Author)

• Good fit for the Rocky Linux project? Title and Author Metatags inserted ?
• If applicable, steps and instructions have been tested to work
• Initial self-review to fix basic typos and grammar completed

Rocky Documentation checklist (Completed by Rocky team)

• 1st Pass (Document is good fit for project and author checklist completed)
• 2nd Pass (Technical Review - check for technical correctness)
• 3rd Pass (Detailed Editorial Review and Peer Review)
• Final approval (Final Review)

[github-actions]

Test results for 69879e6:

Number of broken URLs: 6

URL,RESULT,FILENAME
 https://stigviewer.com/stigs/red_hat_enterprise_linux_8,failed,books/disa_stig/disa_stig_part1.md
 https://stigviewer.com/stigs/apache_server_2.4_unix_server,failed,books/disa_stig/disa_stig_part3.md
 http://$(hostname):8080,failed,guides/repositories/pulp_fetch_upload.md
 https://ftp.gnu.org/gnu/tar/,failed,guides/backup/tar.md
 https://ftp.gnu.org/gnu/hello/hello-2.12.tar.gz,failed,labs/systems_administration_I/lab7-software_management.md
 https://repocompare.rockylinux.org,failed,release_notes/8_5.md