setup: bump tested Ubuntu version

The current dataplane-stack code has been verified on Ubuntu 22.04.
Update the wording in setup.sh to reflect this change.

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Dhruv Tripathi <dhruv.tripathi@arm.com>

Change-Id: Idd4d030ceeb3c98395ea8dbddb912499b0d5ade4