Make disk.threshold_enabled operator only (#78822)

Orchestrated environments should not allow users to override
`cluster.routing.allocation.disk.threshold_enabled`, so making this
operator only.

Closes #77846

server/src/main/java/org/elasticsearch/cluster/routing/allocation/DiskThresholdSettings.java

@@ -30,7 +30,7 @@ import java.util.Map;
 public class DiskThresholdSettings {
     public static final Setting<Boolean> CLUSTER_ROUTING_ALLOCATION_DISK_THRESHOLD_ENABLED_SETTING =
         Setting.boolSetting("cluster.routing.allocation.disk.threshold_enabled", true,
-            Setting.Property.Dynamic, Setting.Property.NodeScope);
+            Setting.Property.OperatorDynamic, Setting.Property.NodeScope);
     public static final Setting<String> CLUSTER_ROUTING_ALLOCATION_LOW_DISK_WATERMARK_SETTING =
         new Setting<>("cluster.routing.allocation.disk.watermark.low", "85%",
             (s) -> validWatermarkSetting(s, "cluster.routing.allocation.disk.watermark.low"),