### 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
long numDocs = randomIntBetween(32, 2048);
        final long intervalMillis = TimeValue.timeValueHours(1).millis();
        long now = System.currentTimeMillis();
        long oneWeekAgo = now - 604800000;
        long twoWeeksAgo = oneWeekAgo - 604800000;
        indexDocs(
            logger,
            indexName,
            1,
            Intervals.alignToCeil(twoWeeksAgo, intervalMillis),
            Intervals.alignToCeil(twoWeeksAgo, intervalMillis) + 1
        );
        indexDocs(
            logger,
            indexName,
            numDocs - 1,
            Intervals.alignToCeil(twoWeeksAgo, intervalMillis) + intervalMillis,
            Intervals.alignToFloor(oneWeekAgo, intervalMillis)
        );
        long numDocs2 = randomIntBetween(32, 2048);
        indexDocs(
            logger,
            indexName,
            numDocs2,
            Intervals.alignToCeil(oneWeekAgo, intervalMillis),
            Intervals.alignToFloor(now, intervalMillis)
        );
        client().admin().cluster().prepareHealth(indexName).setWaitForYellowStatus().get();
        String scrollJobId = "stop-restart-scroll";
        Job.Builder scrollJob = createScheduledJob(scrollJobId);