Update Makefile to use 21 release ISO URLs
Update Makefile to use 21 release ISO URLs

boxcutter/fedora,avtar/packer-fedora,waharnum/packer-fedora,waharnum/packer-fedora,boxcutter/fedora,bertvv/fedora,avtar/packer-fedora,idi-ops/packer-fedora,bertvv/fedora