pungi: update treefile to use for rawhide fedora-bootc
    
    As discussed in https://gitlab.com/fedora/bootc/tracker/-/issues/39,
    we will be maintaining all Fedora versions in the `main` branch going
    forward. The `fedora-bootc.yaml` file upstream is now version-less. To
    not break pungi, we need it to use the stub treefile we added that sets
    the releasever to rawhide.