in this change,
* suites/rados/upgrade: luminous-x-singleton => mimic-x-singleton
* suites/upgrade: luminous-x => nautilus-x
we support upgrade from n to n+2 release. otherwise monitor refuses to
do so:
mon.a@-1(probing) e1 current monmap has min_mon_release 15 (luminous)
which is >2 releases older than me 15 (octopus), stopping.
Fixes: https://tracker.ceph.com/issues/38845
Signed-off-by: Kefu Chai <kchai@redhat.com>