### 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/TransportStartDatafeedAction.java
REMOTE_CLUSTERS_TOO_OLD,
                minVersion.toString(),
                reason,
                Strings.collectionToCommaDelimitedString(clustersTooOld)
            )
        );
    }
    
    private void createDataExtractor(
        Task task,
        Job job,
        DatafeedConfig datafeed,
        StartDatafeedAction.DatafeedParams params,
        ActionListener<PersistentTasksCustomMetadata.PersistentTask<StartDatafeedAction.DatafeedParams>> listener
    ) {
        DataExtractorFactory.create(
            new ParentTaskAssigningClient(client, clusterService.localNode(), task),
            datafeed,
            job,
            xContentRegistry,
            
            new DatafeedTimingStatsReporter(new DatafeedTimingStats(job.getId()), (ts, refreshPolicy) -> {}),
            ActionListener.wrap(
                unused -> persistentTasksService.sendStartRequest(
                    MlTasks.datafeedTaskId(params.getDatafeedId()),
                    MlTasks.DATAFEED_TASK_NAME,
                    params,
                    listener
                ),
                listener::onFailure