It has never been easier to setup Proxmox, Kubernetes, and Actions Runner Controller (ARC) to do your own CI on old hardware you might otherwise recycle.
I don't know why but individual users have to create a single instance runner and mate it with exactly one of their repositories.
Converting from an individual user to a GitHub organization is not simple. The docs certainly make it look that way but I have yet to run into anybody that has done so without some form of catastrophic error that requires support to fix