### 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
);
                        return;
                    }
                }
                jobConfigProvider.getJob(datafeedConfig.getJobId(), null, jobListener);
            } catch (Exception e) {
                listener.onFailure(e);
            }
        }, listener::onFailure);
        datafeedConfigProvider.getDatafeedConfig(params.getDatafeedId(), null, datafeedListener);
    }
    static void checkRemoteClusterVersions(
        DatafeedConfig config,
        List<String> remoteClusters,
        Function<String, Version> clusterVersionSupplier
    ) {
        Optional<Tuple<Version, String>> minVersionAndReason = config.minRequiredClusterVersion();
        if (minVersionAndReason.isPresent() == false) {
            return;
        }
        final String reason = minVersionAndReason.get().v2();
        final Version minVersion = minVersionAndReason.get().v1();
        List<String> clustersTooOld = remoteClusters.stream()
            .filter(cn -> clusterVersionSupplier.apply(cn).before(minVersion))
            .collect(Collectors.toList());
        if (clustersTooOld.isEmpty()) {
            return;
        }
        throw ExceptionsHelper.badRequestException(
            Messages.getMessage(