### 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
ActionListener<ExplainDataFrameAnalyticsAction.Response> explainListener = ActionListener.wrap(explainResponse -> {
            ByteSizeValue expectedMemoryWithoutDisk = explainResponse.getMemoryEstimation().getExpectedMemoryWithoutDisk();
            auditor.info(jobId, Messages.getMessage(Messages.DATA_FRAME_ANALYTICS_AUDIT_ESTIMATED_MEMORY_USAGE, expectedMemoryWithoutDisk));
            
            
            if (startContext.config.getModelMemoryLimit().compareTo(expectedMemoryWithoutDisk) < 0) {
                String warning = Messages.getMessage(
                    Messages.DATA_FRAME_ANALYTICS_AUDIT_ESTIMATED_MEMORY_USAGE_HIGHER_THAN_CONFIGURED,
                    startContext.config.getModelMemoryLimit(),
                    expectedMemoryWithoutDisk
                );
                auditor.warning(jobId, warning);
                logger.warn("[{}] {}", jobId, warning);
                HeaderWarning.addWarning(warning);
            }
            
            memoryTracker.addDataFrameAnalyticsJobMemoryAndRefreshAllOthers(
                jobId,
                startContext.config.getModelMemoryLimit().getBytes(),
                ActionListener.wrap(aVoid -> listener.onResponse(startContext), listener::onFailure)
            );
        }, listener::onFailure);
        ExplainDataFrameAnalyticsAction.Request explainRequest = new ExplainDataFrameAnalyticsAction.Request(startContext.config);
        ClientHelper.executeAsyncWithOrigin(
            client,
            ClientHelper.ML_ORIGIN,
            ExplainDataFrameAnalyticsAction.INSTANCE,
            explainRequest,
            explainListener
        );