Fix Serialization version check in Shutdown Status

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;