Oh, I see.
The net-ssh library we use uses the first line of the private key to determine which type of key (RSA, ed25519 and so on) it is. The version of ssh shipped with EL7 generates keys in the PEM format, where private keys contain the string BEGIN RSA PRIVATE KEY on the first line of the key. The version shipped with EL8 generates keys in RFC4716 which contain BEGIN OPENSSH PRIVATE KEY and which are considered to be ed25519 keys by the library we use. Hence the error is fired because we don’t ship the dependencies it needs for handling ed25519 keys.
If you don’t mind regenerating your keys, then it is easy. Just regenerate them in the right format with the following command and then redeploy the public key to the target hosts.
sudo -u foreman-proxy ssh-keygen -f ~foreman-proxy/.ssh/id_rsa_foreman_proxy -m PEM -N '' -t rsa -b 4096
We will need to be explicit about the format we require in the installer.