rbd-mirror: Remove old non-primary demoted image snapshots on the local cluster

rbd-mirror: Remove old non-primary demoted image snapshots on the local cluster

When an image is demoted on primary cluster and later promoted again,
a non-primary demoted image snapshot is created on the peer (secondary)
cluster. Each such promote/demote cycle on same cluster results in an
additional non-primary demoted snapshot being created on peer.
As a result, repeated promote/demote cycles on the same cluster can
lead to accumulation of stale non-primary demoted snapshots on the
secondary cluster. These snapshots are not removed immediately because
cleanup is only triggered when peer (secondary) is promoted and then
demoted.

The proposed changes ensures that such snapshots are proactively identified
and removed, preventing unbounded buildup. Also added test coverage to
verify cleanup behavior.

Fixes: https://tracker.ceph.com/issues/76155
Signed-off-by: Miki Patel <miki.patel132@gmail.com>