### 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
)
                                )
                            );
                        } else {
                            final RemoteClusterService remoteClusterService = transportService.getRemoteClusterService();
                            List<String> remoteAliases = RemoteClusterLicenseChecker.remoteClusterAliases(
                                remoteClusterService.getRegisteredRemoteClusterNames(),
                                remoteIndices
                            );
                            checkRemoteClusterVersions(
                                datafeedConfigHolder.get(),
                                remoteAliases,
                                (cn) -> remoteClusterService.getConnection(cn).getVersion()
                            );
                            createDataExtractor(task, job, datafeedConfigHolder.get(), params, waitForTaskListener);
                        }
                    },
                        e -> listener.onFailure(
                            createUnknownLicenseError(
                                params.getDatafeedId(),
                                RemoteClusterLicenseChecker.remoteIndices(params.getDatafeedIndices()),
                                e
                            )
                        )
                    )
                );
            } else {
                createDataExtractor(task, job, datafeedConfigHolder.get(), params, waitForTaskListener);
            }
        };