### 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
Thread.sleep(randomIntBetween(1, 9));
        KillProcessAction.Request killRequest = new KillProcessAction.Request(job.getId());
        client().execute(KillProcessAction.INSTANCE, killRequest).actionGet();
        
        
        waitUntilJobIsClosed(job.getId(), TimeValue.timeValueSeconds(2));
    }
    private void startRealtime(String jobId) throws Exception {
        startRealtime(jobId, null);
    }
    private void startRealtime(String jobId, Integer maxEmptySearches) throws Exception {
        client().admin().indices().prepareCreate("data").setMapping("time", "type=date").get();
        long now = System.currentTimeMillis();
        long numDocs1;
        if (maxEmptySearches == null) {
            numDocs1 = randomIntBetween(32, 2048);
            long lastWeek = now - 604800000;
            indexDocs(logger, "data", numDocs1, lastWeek, now);
        } else {
            numDocs1 = 0;
        }
        Job.Builder job = createScheduledJob(jobId);
        putJob(job);
        openJob(job.getId());
        assertBusy(() -> assertEquals(getJobStats(job.getId()).get(0).getState(), JobState.OPENED));
        DatafeedConfig.Builder datafeedConfigBuilder = createDatafeedBuilder(
            job.getId() + "-datafeed",