mergify: add stable-6.0 backport configuration
mergify: add stable-6.0 backport configuration

This adds the stable-6.0 backport configuration in mergify.

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible