DOCSP-45645 RHEL 9 Support (#575)

Author: kennethdyer

source/installation/install-on-linux.txt

@@ -43,6 +43,7 @@ systems:
 - Amazon Linux 2
 - Red Hat Enterprise Linux (RHEL) 7
 - Red Hat Enterprise Linux (RHEL) 8
+- Red Hat Enterprise Linux (RHEL) 9
 - Ubuntu 18.04
 - Ubuntu 20.04