### Description

This applies to the following APIs:
- [x] open anomaly detection
- [x] start datafeed
- [x] start data frame analytics job
- [x] start trained model deployment

Currently, if the operation takes longer than the configured `timeout`, the response status code is `500`.

With Kibana in mind, a `4xx` response would be more suitable as it will lead to showing more useful information to the user about why the request failed.

Looking at [RFC 9110](https://www.rfc-editor.org/rfc/rfc9110.html#name-408-request-timeout), it seems the most appropriate `4xx` code would be `408 Request Timeout`, even though the RFC describes a different situation. 

The alternative, `504 Gateway Timeout`, would be more appropriate for this situation but it wouldn't improve the user experience in Kibana.
x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/action/TransportOpenJobAction.java
if (predicate.exception != null) {
                        if (predicate.shouldCancel) {
                            
                            
                            cancelJobStart(persistentTask, predicate.exception, listener);
                        } else {
                            listener.onFailure(predicate.exception);
                        }
                    } else {
                        listener.onResponse(new NodeAcknowledgedResponse(true, predicate.node));
                    }
                }
                @Override
                public void onFailure(Exception e) {
                    listener.onFailure(e);
                }
                @Override
                public void onTimeout(TimeValue timeout) {
                    listener.onFailure(new ElasticsearchException("Opening job [{}] timed out after [{}]", jobParams.getJob(), timeout));
                }
            }
        );
    }
    private void clearJobFinishedTime(
        NodeAcknowledgedResponse response,
        ClusterState clusterState,
        String jobId,
        TimeValue masterNodeTimeout,
        ActionListener<NodeAcknowledgedResponse> listener
    ) {