### 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/qa/native-multi-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/ml/integration/DatafeedJobsIT.java
final String datafeedId = jobId + "-datafeed";
        startRealtime(jobId);
        Map<Long, AssertionError> exceptions = ConcurrentCollections.newConcurrentMap();
        
        
        
        Runnable stopDataFeed = () -> {
            StopDatafeedAction.Response stopJobResponse = stopDatafeed(datafeedId);
            if (stopJobResponse.isStopped() == false) {
                exceptions.put(Thread.currentThread().getId(), new AssertionError("Job is not stopped"));
            }
            GetDatafeedsStatsAction.Request request = new GetDatafeedsStatsAction.Request(datafeedId);
            GetDatafeedsStatsAction.Response response = client().execute(GetDatafeedsStatsAction.INSTANCE, request).actionGet();
            if (response.getResponse().results().get(0).getDatafeedState() != DatafeedState.STOPPED) {
                exceptions.put(
                    Thread.currentThread().getId(),
                    new AssertionError("Expected STOPPED datafeed state got " + response.getResponse().results().get(0).getDatafeedState())
                );
            }
        };
        
        
        
        int numThreads = 5;
        Thread[] threads = new Thread[numThreads];
        for (int i = 0; i < numThreads; i++) {
            threads[i] = new Thread(stopDataFeed);
        }
        for (int i = 0; i < numThreads; i++) {
            threads[i].start();