### 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
final JobUpdate update = new JobUpdate.Builder(jobId).setClearFinishTime(true).build();
        ActionListener<Job> clearedTimeListener = ActionListener.wrap(job -> listener.onResponse(response), e -> {
            logger.error(() -> "[" + jobId + "] Failed to clear finished_time", e);
            
            listener.onResponse(response);
        });
        ActionListener<Boolean> mappingsUpdatedListener = ActionListener.wrap(
            mappingUpdateResponse -> jobConfigProvider.updateJob(jobId, update, null, clearedTimeListener),
            e -> {
                logger.error(() -> "[" + jobId + "] Failed to update mapping; not clearing finished_time", e);
                
                listener.onResponse(response);
            }
        );
        ElasticsearchMappings.addDocMappingIfMissing(
            MlConfigIndex.indexName(),
            MlConfigIndex::mapping,
            client,
            clusterState,
            masterNodeTimeout,
            mappingsUpdatedListener
        );
    }
    private void cancelJobStart(
        PersistentTasksCustomMetadata.PersistentTask<OpenJobAction.JobParams> persistentTask,
        Exception exception,
        ActionListener<NodeAcknowledgedResponse> listener
    ) {
        persistentTasksService.sendRemoveRequest(persistentTask.getId(), new ActionListener<>() {
            @Override