Change snapshot entity delimiter when pinning the timestamp

Signed-off-by: Anshu Agarwal <anshukag@amazon.com>

Author: Anshu Agarwal

server/src/main/java/org/opensearch/snapshots/SnapshotsService.java

@@ -204,7 +204,7 @@ public class SnapshotsService extends AbstractLifecycleComponent implements Clus
         Setting.Property.Dynamic
     );
 
-    private static final String SNAPSHOT_PINNED_TIMESTAMP_DELIMITER = ":";
+    private static final String SNAPSHOT_PINNED_TIMESTAMP_DELIMITER = "__";
     private volatile int maxConcurrentOperations;
 
     public SnapshotsService(