rbd-mirror: prune obsolete primary mirror snapshots after relocation

rbd-mirror: prune obsolete primary mirror snapshots after relocation

Previously, obsolete primary and demoted primary snapshots on the
secondary cluster were not cleaned up immediately after relocation.
Instead, old primary snapshots remained until a subsequent promote
operation triggered their cleanup, while old demoted primary snapshots
persisted until a later demote operation removed them.

Adding changes for proactive cleanup of obsolete primary and demoted
primary snapshots that are no longer required after relocation.

Also adding test coverage to validate the cleanup behavior.

Fixes: https://tracker.ceph.com/issues/76154
Signed-off-by: Miki Patel <miki.patel132@gmail.com>