### 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/TransportOpenJobAction.java
private final JobConfigProvider jobConfigProvider;
    private final MlMemoryTracker memoryTracker;
    private final Client client;
    @Inject
    public TransportOpenJobAction(
        TransportService transportService,
        ThreadPool threadPool,
        XPackLicenseState licenseState,
        ClusterService clusterService,
        PersistentTasksService persistentTasksService,
        ActionFilters actionFilters,
        IndexNameExpressionResolver indexNameExpressionResolver,
        JobConfigProvider jobConfigProvider,
        MlMemoryTracker memoryTracker,
        Client client
    ) {
        super(
            OpenJobAction.NAME,
            transportService,
            clusterService,
            threadPool,
            actionFilters,
            OpenJobAction.Request::new,
            indexNameExpressionResolver,
            NodeAcknowledgedResponse::new,
            ThreadPool.Names.SAME
        );
        this.licenseState = licenseState;
        this.persistentTasksService = persistentTasksService;
        this.jobConfigProvider = jobConfigProvider;