Now that mkosi has centos-stream 9 support, let's add a config in the repo so that the mkosi CI tests that configuration as well. Centos doesn't support btrfs so we use xfs instead. For some reason, building --hostonly-initrd centos images breaks the qemu boot so I disabled that option for centos. We update the mkosi commit hash to https://github.com/systemd/mkosi/commit/0dd39c20a4b3a2fab6efdc54da92bffad7c7b7ca which adds the PowerTools repo to CentOS Stream 8 which is required to make all the necessary packages required to build systemd on CentOS Stream 8 available.