Just install one of the old supported OSes on another drive, reboot and run the firmware update. It's a self extracting executable.
With HP and Dell, it's their intention to support you by providing an additional layer of management on top of the actual manufacturers product. But you have to be in warranty to be able to leverage that support. If you intend to run equipment longer than the 5 year warranty, don't use Dell or HP or live without any firmware upgrades or support. If you use Dell or HP, replace hardware when it's out of warranty.
HP Gen8 servers are from 2012-2013 so 7-8 years old and not the best candidates for new installations.