### 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
}
        for (int i = 0; i < numThreads; i++) {
            threads[i].join();
        }
        if (exceptions.isEmpty() == false) {
            throw exceptions.values().iterator().next();
        }
    }
    public void testRealtime_givenSimultaneousStopAndForceDelete() throws Throwable {
        String jobId = "realtime-job-stop-and-force-delete";
        final String datafeedId = jobId + "-datafeed";
        startRealtime(jobId);
        AtomicReference<Throwable> exception = new AtomicReference<>();
        
        
        
        
        
        Thread deleteDatafeedThread = new Thread(() -> {
            try {
                DeleteDatafeedAction.Request request = new DeleteDatafeedAction.Request(datafeedId);
                request.setForce(true);
                AcknowledgedResponse response = client().execute(DeleteDatafeedAction.INSTANCE, request).actionGet();
                if (response.isAcknowledged()) {
                    GetDatafeedsStatsAction.Request statsRequest = new GetDatafeedsStatsAction.Request(datafeedId);
                    expectThrows(
                        ResourceNotFoundException.class,
                        () -> client().execute(GetDatafeedsStatsAction.INSTANCE, statsRequest).actionGet()
                    );
                } else {