Updating documentation (specifically removing reference to Centos7)

Wasn’t sure whether to just put in a pull request to the documentation Github repo or discuss here first but I see mention of Centos7 a lot in the documentation which is now retired I believe. Just going through upgrade to AlmaLinux 9 so just refreshing memory and checking nothing changed and noticed this. Another example https://yum.osc.edu/ondemand/3.1/ondemand-release-web-3.1-1.el7.noarch.rpm doesn’t exist but mentioned in docs.

I’ll take a look at the docs this week to remove all references to centos 7.