unattended-installation,rhel,fedora: Use "inst.ks=" instead of "ks="
"ks=" is supposed to be depecrated soon, and it's been causing some harmless warnings in the installation. Before it actually gets deprecated, let's already do the switch to using "inst.ks=" instead for Fedora 33+ and RHEL 8+.
All released RHEL 8 support "inst.ks=". :-)