### 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/src/main/java/org/elasticsearch/xpack/ml/action/TransportStartDatafeedAction.java
private static final Version COMPOSITE_AGG_SUPPORT = Version.V_7_13_0;
    private static final Logger logger = LogManager.getLogger(TransportStartDatafeedAction.class);
    private final Client client;
    private final XPackLicenseState licenseState;
    private final PersistentTasksService persistentTasksService;
    private final JobConfigProvider jobConfigProvider;
    private final DatafeedConfigProvider datafeedConfigProvider;
    private final AnomalyDetectionAuditor auditor;
    private final NamedXContentRegistry xContentRegistry;
    private final boolean remoteClusterClient;
    @Inject
    public TransportStartDatafeedAction(
        Settings settings,
        TransportService transportService,
        ThreadPool threadPool,
        ClusterService clusterService,