### 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
+ "], because the data frame analytics requires a node of version ["
                    + TaskParams.VERSION_INTRODUCED
                    + "] or higher";
            }
            if (node.getVersion().before(TaskParams.VERSION_DESTINATION_INDEX_MAPPINGS_CHANGED)
                && params.getVersion().onOrAfter(TaskParams.VERSION_DESTINATION_INDEX_MAPPINGS_CHANGED)) {
                return "Not opening job ["
                    + id
                    + "] on node ["
                    + JobNodeSelector.nodeNameAndVersion(node)
                    + "], because the data frame analytics created for version ["
                    + params.getVersion()
                    + "] requires a node of version "
                    + "["
                    + TaskParams.VERSION_DESTINATION_INDEX_MAPPINGS_CHANGED
                    + "] or higher";
            }
            return null;
        }
        @Override
        protected String[] indicesOfInterest(TaskParams params) {
            return new String[] { MlConfigIndex.indexName(), MlStatsIndex.indexPattern(), AnomalyDetectorsIndex.jobStateIndexPattern() };
        }
        @Override
        protected String getJobId(TaskParams params) {
            return params.getId();
        }
    }
}