### 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 volatile boolean shouldCancel;
        @Override
        public boolean test(PersistentTasksCustomMetadata.PersistentTask<?> persistentTask) {
            JobState jobState = JobState.CLOSED;
            String reason = null;
            if (persistentTask != null) {
                JobTaskState jobTaskState = (JobTaskState) persistentTask.getState();
                jobState = jobTaskState == null ? JobState.OPENING : jobTaskState.getState();
                reason = jobTaskState == null ? null : jobTaskState.getReason();
                PersistentTasksCustomMetadata.Assignment assignment = persistentTask.getAssignment();
                
                if (assignment != null && assignment.equals(JobNodeSelector.AWAITING_LAZY_ASSIGNMENT)) {
                    return true;
                }
                
                
                OpenJobAction.JobParams params = (OpenJobAction.JobParams) persistentTask.getParams();
                Optional<ElasticsearchException> assignmentException = checkAssignmentState(assignment, params.getJobId(), logger);
                if (assignmentException.isPresent()) {
                    exception = assignmentException.get();
                    
                    
                    shouldCancel = true;
                    return true;
                }
            }
            switch (jobState) {