Yeah, it would require a config file change in the sources file. Distribution is part of how the URL is constructed.
deb http://us.archive.ubuntu.com/ubuntu focal main
In this focal is the distribution, main is the component (of which there can be several). Changing the name of the distribution in the repo requires a change in the sources.list.
I recommended webmin for distribution (and main for component, though that matters less) back when we were discussing it. That said, I’m surprised (unpleasantly so) that anything is complaining about it. I don’t think it is any of Proxmoxes business what a distribution is called; I don’t believe Debian or Ubuntu has a policy about this. I think they just invented it.
If there is a policy about it, I think it’s probably worth us changing. But, if there isn’t a policy, Proxmox should probably shut up about it. I’d welcome a link, if there is such a policy.