Include targetNodeName in shutdown metadata toString()

Reporting the `targetNodeName` was added to `main` in elastic#78727 but omitted
from the backport in elastic#78865. This commit adds the missing field to the
`toString()` response.

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

@@ -271,6 +271,9 @@ public String toString() {
         if (allocationDelay != null) {
             stringBuilder.append(", allocationDelay=[").append(allocationDelay).append("]");
         }
+        if (targetNodeName != null) {
+            stringBuilder.append(", targetNodeName=[").append(targetNodeName).append("]");
+        }
         stringBuilder.append("}");
         return stringBuilder.toString();
     }