Being a G5, this blade is long out of support by HP, meaning there won't likely be any further BIOS updates. You say you went and got the latest, but that's still dated December 2010.
This hardware doesn't support Windows 2012, and that won't change. I'm expecting there's a flow on effect that a microcode update for Windows 2012 isn't likely to be seen.
I can only suggest ignoring the error, and considering it as part of a case for upgrades.