Update recommended OS version for building SONiC (#22846)

Ubuntu 20.04 has gone EOL. Ubuntu 22.04 is in standard support until April 2027. 22.04 is also the version used in the sonic-buildimage repo's Azure Pipelines builds

Fixes #22704

Why I did it
Ubuntu 20.04 is EOL so shouldn't be recommended.

Author: nate-nexthop

README.md

@@ -80,7 +80,7 @@ Any server can be a build image server as long as it has:
 > options to expose the full KVM interface to the VM
 > (e.g. [the KVM paravirtualization support on VirtualBox](https://www.virtualbox.org/manual/ch10.html#gimproviders)).
 
-A good choice of OS for building SONiC is currently Ubuntu 20.04.
+A good choice of OS for building SONiC is currently Ubuntu 22.04.
 
 ## Prerequisites