### 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
putJob(scrollJob);
        openJob(scrollJobId);
        assertBusy(() -> assertEquals(getJobStats(scrollJobId).get(0).getState(), JobState.OPENED));
        DatafeedConfig datafeedConfig = createDatafeedBuilder(scrollJobId + "-datafeed", scrollJobId, Collections.singletonList(indexName))
            .setChunkingConfig(ChunkingConfig.newManual(new TimeValue(1, TimeUnit.SECONDS)))
            .build();
        putDatafeed(datafeedConfig);
        startDatafeed(datafeedConfig.getId(), 0L, null);
        
        assertBusy(() -> assertThat(getDataCounts(scrollJobId).getProcessedRecordCount(), greaterThan(0L)));
        stopDatafeed(datafeedConfig.getId());
        assertBusy(() -> assertThat(getJobStats(scrollJobId).get(0).getState(), is(oneOf(JobState.CLOSED, JobState.OPENED))));
        
        if (getJobStats(scrollJobId).get(0).getState().equals(JobState.OPENED)) {
            updateDatafeed(new DatafeedUpdate.Builder().setId(datafeedConfig.getId()).setChunkingConfig(ChunkingConfig.newAuto()).build());
            startDatafeed(
                datafeedConfig.getId(),
                randomLongBetween(0, getDataCounts(scrollJobId).getLatestRecordTimeStamp().getTime()),
                now
            );
            waitUntilJobIsClosed(scrollJobId);
        }
        assertBusy(() -> {
            DataCounts dataCounts = getDataCounts(scrollJobId);
            assertThat(dataCounts.getProcessedRecordCount(), equalTo(numDocs + numDocs2));
            assertThat(dataCounts.getOutOfOrderTimeStampCount(), equalTo(0L));
        }, 60, TimeUnit.SECONDS);
        String compositeJobId = "stop-restart-composite";
        Job.Builder compositeJob = createScheduledJob(compositeJobId);
        compositeJob.setAnalysisConfig(new AnalysisConfig.Builder(compositeJob.getAnalysisConfig()).setSummaryCountFieldName("doc_count"));