Fix Serialization version check in Shutdown Status (elastic#81342)

The version used to determine whether we should serialize the allocation
decisions as part of the Shutdown Status response was not updated after
elastic#78727 was merged. This causes very confusing errors in mixed 8.x/7.16.x
clusters. Note: The version constant was updated in the backport to
7.16, but not in 8.x. This is good, as at time of writing, 7.16 has
shipped, but 8.0 has not.

server/src/main/java/org/elasticsearch/cluster/metadata/ShutdownShardMigrationStatus.java

@@ -22,7 +22,7 @@
 import java.util.Objects;
 
 public class ShutdownShardMigrationStatus implements Writeable, ToXContentObject {
-    private static final Version ALLOCATION_DECISION_ADDED_VERSION = Version.V_8_0_0;
+    private static final Version ALLOCATION_DECISION_ADDED_VERSION = Version.V_7_16_0;
 
     private final SingleNodeShutdownMetadata.Status status;
     private final long shardsRemaining;