I've installed ESXi 7u3 (and 8, for that matter) on legacy unsupported servers, including a Dell R720 (12th gen, 2012-2013 server, for reference) in my homelab, and a SuperMicro server at work (probably made in that same 2013 time period) with no issues. I get warnings about unsupported CPU, but that can be bypassed. I think the HP Gen8 servers were made in 2013-2014, so you'd probably be OK running at least ESXi 7.
