installation procedure not up-to-date

[PhilippePinheiroLIST]

unix installation procedure seems outdated concerning the pre-installation procedure, it stills refers to php5 :

https://www.mapserver.org/installation/unix.html#installation

but here I found a most recent doc on the github wiki also working on ubuntu 20.04 👍

https://github.com/mapserver/mapserver/wiki/Install-mapserver-on-ubuntu-18.04-from-official-repository

updating mapserver doc may help new users installing it

[jmckenna]

@PhilippePinheiroLIST are you able to create a pull request with your changes? (please make it against the 'main' branch) thanks!