Fix Missing IgnoredUnavailable Flag in 7.x SLM Retention Task (#56616)

Without the flag we run into the situation where a broken repository (broken by some old 6.x
version of ES that is missing some snap-${uuid}.dat blobs fails to run the SLM retention task
since it always errors out).

x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/slm/SnapshotRetentionTask.java

@@ -270,6 +270,7 @@ public class SnapshotRetentionTask implements SchedulerEngine.Listener {
             for (String repository : repositories) {
                 client.admin().cluster()
                     .prepareGetSnapshots(repository)
+                    .setIgnoreUnavailable(true)
                     .execute(ActionListener.wrap(resp -> {
                             final Set<SnapshotState> retainableStates =
                                 new HashSet<>(Arrays.asList(SnapshotState.SUCCESS, SnapshotState.FAILED, SnapshotState.PARTIAL));