### 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
return state.blocks().globalBlockedException(ClusterBlockLevel.METADATA_WRITE);
    }
    @Override
    protected void masterOperation(
        Task task,
        StartDataFrameAnalyticsAction.Request request,
        ClusterState state,
        ActionListener<NodeAcknowledgedResponse> listener
    ) {
        logger.debug(() -> "[" + request.getId() + "] received start request");
        if (MachineLearningField.ML_API_FEATURE.check(licenseState) == false) {
            listener.onFailure(LicenseUtils.newComplianceException(XPackField.MACHINE_LEARNING));
            return;
        }
        
        ActionListener<PersistentTasksCustomMetadata.PersistentTask<TaskParams>> waitForAnalyticsToStart = new ActionListener<
            PersistentTasksCustomMetadata.PersistentTask<TaskParams>>() {
            @Override
            public void onResponse(PersistentTasksCustomMetadata.PersistentTask<TaskParams> task) {
                waitForAnalyticsStarted(task, request.getTimeout(), listener);
            }
            @Override
            public void onFailure(Exception e) {
                if (ExceptionsHelper.unwrapCause(e) instanceof ResourceAlreadyExistsException) {
                    e = new ElasticsearchStatusException(
                        "Cannot start data frame analytics [{}] because it has already been started",
                        RestStatus.CONFLICT,
                        e,
                        request.getId()