### 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
assert enabled(environment);
        try {
            logger.info(
                "stopping readiness service on channel {}",
                (this.serverChannel == null) ? "None" : this.serverChannel.getLocalAddress()
            );
            if (this.serverChannel != null) {
                this.serverChannel.close();
                listenerThreadLatch.await();
            }
        } catch (InterruptedException | IOException e) {
            logger.warn("error closing readiness service channel", e);
        } finally {
            this.serverChannel = null;
            logger.info("readiness service stopped");
        }
    }
    @Override
    protected void doClose() {}
    @Override
    public void clusterChanged(ClusterChangedEvent event) {
        ClusterState clusterState = event.state();
        Set<String> shutdownNodeIds = PluginShutdownService.shutdownNodes(clusterState);
        if (shutdownNodeIds.contains(clusterState.nodes().getLocalNodeId())) {
            setReady(false);
            logger.info("marking node as not ready because it's shutting down");
        } else {
            setReady(clusterState.nodes().getMasterNodeId() != null);
        }
    }