### 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
Collections.singletonList(
                        new DateHistogramValuesSourceBuilder("timebucket").fixedInterval(new DateHistogramInterval("1h")).field("time")
                    )
                ).subAggregation(AggregationBuilders.max("time").field("time")).size(100)
            );
            updateDatafeed(new DatafeedUpdate.Builder().setId(compositeDatafeedConfig.getId()).setParsedAggregations(aggs).build());
            startDatafeed(
                compositeDatafeedConfig.getId(),
                randomLongBetween(0, getDataCounts(compositeJobId).getLatestRecordTimeStamp().getTime()),
                now
            );
            waitUntilJobIsClosed(compositeJobId);
        }
        List<Bucket> scrollBuckets = getBuckets(scrollJobId);
        List<Bucket> compositeBuckets = getBuckets(compositeJobId);
        assertThat(
            "scroll bucket size " + scrollBuckets + " does not equal composite bucket size" + compositeBuckets,
            compositeBuckets.size(),
            equalTo(scrollBuckets.size())
        );
        for (int i = 0; i < scrollBuckets.size(); i++) {
            Bucket scrollBucket = scrollBuckets.get(i);
            Bucket compositeBucket = compositeBuckets.get(i);
            try {
                assertThat(
                    "scroll buckets " + scrollBuckets + " composite buckets " + compositeBuckets,
                    compositeBucket.getTimestamp(),
                    equalTo(scrollBucket.getTimestamp())
                );
                assertThat(