Use Snapshot Bulk Delete in SLM's SnapshotRetentionTask

[original-brownbear]

#55474 introduced bulk deletes for snapshots.
We should use it in org.elasticsearch.xpack.slm.SnapshotRetentionTask#deleteSnapshots to speed up the retention task in case multiple snapshots have to be deleted.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/ILM+SLM)