Set RemoteClusterPortSettings.PORT default to 9400 in stateless (#132507)

Serverless cross-project search will enable the remote cluster
server by default on search nodes.  This PR sets the default
"remote_cluster.port" setting value to 9400 for stateless.

Relates: ES-12587

Author: JeremyDahlgren

server/src/main/java/org/elasticsearch/transport/RemoteClusterPortSettings.java

@@ -11,6 +11,7 @@
 
 import org.elasticsearch.TransportVersion;
 import org.elasticsearch.TransportVersions;
+import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.ByteSizeUnit;
@@ -70,7 +71,13 @@ public class RemoteClusterPortSettings {
         Setting.Property.NodeScope
     );
 
-    public static final Setting<Integer> PORT = intSetting(REMOTE_CLUSTER_PREFIX + "port", 9443, 0, 65535, Setting.Property.NodeScope);
+    public static final Setting<Integer> PORT = intSetting(
+        REMOTE_CLUSTER_PREFIX + "port",
+        (settings) -> DiscoveryNode.isStateless(settings) ? "9400" : "9443",
+        0,
+        65535,
+        Setting.Property.NodeScope
+    );
 
     // The default value of -1 means it will use the default bind port as shown above
     public static final Setting<Integer> PUBLISH_PORT = intSetting(

server/src/test/java/org/elasticsearch/transport/RemoteClusterPortSettingsTests.java

@@ -9,6 +9,7 @@
 
 package org.elasticsearch.transport;
 
+import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.common.network.NetworkAddress;
 import org.elasticsearch.common.network.NetworkService;
 import org.elasticsearch.common.settings.Setting;
@@ -204,4 +205,11 @@ public void testPortSettingsConstruction() {
         assertThat(profileSettings.isDefaultProfile, equalTo(false));
     }
 
+    public void testRemoteClusterPortDefaultValue() {
+        assertThat(RemoteClusterPortSettings.PORT.getDefault(Settings.EMPTY), equalTo(9443));
+        assertThat(
+            RemoteClusterPortSettings.PORT.getDefault(Settings.builder().put(DiscoveryNode.STATELESS_ENABLED_SETTING_NAME, true).build()),
+            equalTo(9400)
+        );
+    }
 }