### 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
static void validate(
        Job job,
        DatafeedConfig datafeedConfig,
        PersistentTasksCustomMetadata tasks,
        NamedXContentRegistry xContentRegistry
    ) {
        DatafeedJobValidator.validate(datafeedConfig, job, xContentRegistry);
        DatafeedConfig.validateAggregations(datafeedConfig.getParsedAggregations(xContentRegistry));
        JobState jobState = MlTasks.getJobState(datafeedConfig.getJobId(), tasks);
        if (jobState.isAnyOf(JobState.OPENING, JobState.OPENED) == false) {
            throw ExceptionsHelper.conflictStatusException(
                "cannot start datafeed [" + datafeedConfig.getId() + "] because job [" + job.getId() + "] is " + jobState
            );
        }
    }
    
    static void auditDeprecations(
        DatafeedConfig datafeed,
        Job job,
        AnomalyDetectionAuditor auditor,
        NamedXContentRegistry xContentRegistry
    ) {
        List<String> deprecationWarnings = new ArrayList<>();
        deprecationWarnings.addAll(datafeed.getAggDeprecations(xContentRegistry));
        deprecationWarnings.addAll(datafeed.getQueryDeprecations(xContentRegistry));
        if (deprecationWarnings.isEmpty() == false) {
            String msg = "datafeed ["
                + datafeed.getId()
                + "] configuration has deprecations. ["
                + Strings.collectionToDelimitedString(deprecationWarnings, ", ")