Add latest dropbear with OpenSSH host key format support

Our previous initrd contained dropbear release dropbear-2020.81 which did support ed25519 keys but did not yet support the new OpenSSH host key format. We had to use debian 9 or explicitly generate PEM formatted keys to work around the compatibility issue. The latest release 2022.82 includes OpenSSH host key format support so we can keep our instance generation more generic and up-to-date.