### 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/TransportStartDataFrameAnalyticsAction.java
super(
            StartDataFrameAnalyticsAction.NAME,
            transportService,
            clusterService,
            threadPool,
            actionFilters,
            StartDataFrameAnalyticsAction.Request::new,
            indexNameExpressionResolver,
            NodeAcknowledgedResponse::new,
            ThreadPool.Names.SAME
        );
        this.licenseState = licenseState;
        this.client = client;
        this.persistentTasksService = persistentTasksService;
        this.configProvider = configProvider;
        this.memoryTracker = memoryTracker;
        this.auditor = Objects.requireNonNull(auditor);
        this.sourceDestValidator = new SourceDestValidator(
            indexNameExpressionResolver,
            transportService.getRemoteClusterService(),
            null,
            null,
            clusterService.getNodeName(),
            License.OperationMode.PLATINUM.description()
        );
    }
    @Override
    protected ClusterBlockException checkBlock(StartDataFrameAnalyticsAction.Request request, ClusterState state) {