### Description

The new [readiness port](https://www.elastic.co/guide/en/elasticsearch/reference/8.4/advanced-configuration.html#readiness-tcp-port) makes readiness checks on Kubernetes much easier. However the current implementation only allows connections through localhost. TCP probes on Kubernetes are run by the kubelet which is a process running on the Kubernetes node (and not from within the container or the Pod). The [Kubernetes documentation](https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#tcp-probes) mentions it, but it is kind of subtle: 

> For a TCP probe, the kubelet makes the probe connection at the node, not in the pod, which means that you can not use a service name in the host parameter since the kubelet is unable to resolve it.


That means a standard Kubernetes readiness probe cannot be used for Elasticsearch:
```
          readinessProbe:
            tcpSocket:
              port: 8080
```

Instead, only an `exec` probe that runs a shell command inside the container can be used that relies in this case on the presence of the `nc` command in the container image:

```
    readinessProbe:
      exec:
        command: ["nc", "-z", "-v", "-w5", "127.0.0.1", "8080"] 
```



server/src/main/java/org/elasticsearch/readiness/ReadinessService.java
private static final Logger logger = LogManager.getLogger(ReadinessService.class);
    private final Environment environment;
    private volatile boolean active; 
    private volatile ServerSocketChannel serverChannel;
    
    volatile CountDownLatch listenerThreadLatch = new CountDownLatch(0);
    final AtomicReference<InetSocketAddress> boundSocket = new AtomicReference<>();
    private final Collection<BoundAddressListener> boundAddressListeners = new CopyOnWriteArrayList<>();
    public static final Setting<Integer> PORT = Setting.intSetting("readiness.port", -1, Setting.Property.NodeScope);
    public ReadinessService(ClusterService clusterService, Environment environment) {
        this.serverChannel = null;
        this.environment = environment;
        clusterService.addListener(this);
    }
    
    boolean ready() {
        return this.serverChannel != null;
    }
    /**
     * Checks to see if the readiness service is enabled in the current environment
     * @param environment
     * @return
     */
    public static boolean enabled(Environment environment) {
        return PORT.get(environment.settings()) != -1;
    }
    
    ServerSocketChannel serverChannel() {
        return serverChannel;
    }