### 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/MlJobIT.java
}
            
            
            if (recreationGuard.getAndIncrement() == 0) {
                try {
                    recreationResponse.set(createFarequoteJob(jobId));
                } catch (ResponseException re) {
                    recreationException.set(re);
                } catch (IOException e) {
                    logger.error("Error trying to recreate the job", e);
                    ioe.set(e);
                }
            }
        };
        
        
        
        int numThreads = 5;
        Thread[] threads = new Thread[numThreads];
        for (int i = 0; i < numThreads; i++) {
            threads[i] = new Thread(deleteJob);
        }
        for (int i = 0; i < numThreads; i++) {
            threads[i].start();
        }
        for (int i = 0; i < numThreads; i++) {
            threads[i].join();
        }
        if (ioe.get() != null) {