### 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/TransportStartDataFrameAnalyticsAction.java
private void cancelAnalyticsStart(
        PersistentTasksCustomMetadata.PersistentTask<TaskParams> persistentTask,
        Exception exception,
        ActionListener<NodeAcknowledgedResponse> listener
    ) {
        persistentTasksService.sendRemoveRequest(
            persistentTask.getId(),
            new ActionListener<PersistentTasksCustomMetadata.PersistentTask<?>>() {
                @Override
                public void onResponse(PersistentTasksCustomMetadata.PersistentTask<?> task) {
                    
                    
                    listener.onFailure(exception);
                }
                @Override
                public void onFailure(Exception e) {
                    logger.error(
                        () -> format(
                            "[%s] Failed to cancel persistent task that could not be assigned due to [%s]",
                            persistentTask.getParams().getId(),
                            exception.getMessage()
                        ),
                        e
                    );
                    listener.onFailure(exception);
                }
            }
        );
    }
    public static class TaskExecutor extends AbstractJobPersistentTasksExecutor<TaskParams> {