### 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
+ "]";
            auditor.warning(job.getId(), msg);
        }
    }
    @Override
    protected void masterOperation(
        Task task,
        StartDatafeedAction.Request request,
        ClusterState state,
        ActionListener<NodeAcknowledgedResponse> listener
    ) {
        StartDatafeedAction.DatafeedParams params = request.getParams();
        if (MachineLearningField.ML_API_FEATURE.check(licenseState) == false) {
            listener.onFailure(LicenseUtils.newComplianceException(XPackField.MACHINE_LEARNING));
            return;
        }
        AtomicReference<DatafeedConfig> datafeedConfigHolder = new AtomicReference<>();
        PersistentTasksCustomMetadata tasks = state.getMetadata().custom(PersistentTasksCustomMetadata.TYPE);
        ActionListener<PersistentTasksCustomMetadata.PersistentTask<StartDatafeedAction.DatafeedParams>> waitForTaskListener =
            new ActionListener<>() {
                @Override
                public void onResponse(PersistentTasksCustomMetadata.PersistentTask<StartDatafeedAction.DatafeedParams> persistentTask) {
                    waitForDatafeedStarted(persistentTask.getId(), params, listener);
                }
                @Override
                public void onFailure(Exception e) {
                    if (ExceptionsHelper.unwrapCause(e) instanceof ResourceAlreadyExistsException) {
                        logger.debug("datafeed already started", e);
                        e = new ElasticsearchStatusException(
                            "cannot start datafeed [" + params.getDatafeedId() + "] because it has already been started",