### Description

I noticed that when I run `GET my_ilm_index/_ilm/explain`, the order of the returned JSON seems without much logic.

In my case, we have 18 underlying indices, and they are returned as
```
my_ilm_index-000010
my_ilm_index-000011
my_ilm_index-000012
my_ilm_index-000001
my_ilm_index-000006
my_ilm_index-000017
my_ilm_index-000018
my_ilm_index-000007
my_ilm_index-000008
my_ilm_index-000009
my_ilm_index-000002
my_ilm_index-000013
my_ilm_index-000014
my_ilm_index-000003
my_ilm_index-000015
my_ilm_index-000004
my_ilm_index-000016
my_ilm_index-000005
```

While of course, strictly speaking, json has no order, it's still inconvenient to just get a quick look at what's happening.

Could this get an ordering?

**Details:**
Elastic 7.16.2 in docker
Kibana 7.16.2 in docker
Request done via Kibana/Dev tools/console (on Chrome, on Windows 10)
x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/ilm/action/TransportExplainLifecycleAction.java
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.bytes.BytesArray;
import org.elasticsearch.common.inject.Inject;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.core.Nullable;
import org.elasticsearch.tasks.Task;
import org.elasticsearch.threadpool.ThreadPool;
import org.elasticsearch.transport.TransportService;
import org.elasticsearch.xcontent.NamedXContentRegistry;
import org.elasticsearch.xcontent.XContentParser;
import org.elasticsearch.xcontent.XContentParserConfiguration;
import org.elasticsearch.xcontent.json.JsonXContent;
import org.elasticsearch.xpack.core.ilm.ErrorStep;
import org.elasticsearch.xpack.core.ilm.ExplainLifecycleRequest;
import org.elasticsearch.xpack.core.ilm.ExplainLifecycleResponse;
import org.elasticsearch.xpack.core.ilm.IndexLifecycleExplainResponse;
import org.elasticsearch.xpack.core.ilm.PhaseExecutionInfo;
import org.elasticsearch.xpack.core.ilm.action.ExplainLifecycleAction;
import org.elasticsearch.xpack.ilm.IndexLifecycleService;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import static org.elasticsearch.xpack.core.ilm.LifecycleSettings.LIFECYCLE_ORIGINATION_DATE;
public class TransportExplainLifecycleAction extends TransportClusterInfoAction<ExplainLifecycleRequest, ExplainLifecycleResponse> {
    private final NamedXContentRegistry xContentRegistry;
    private final IndexLifecycleService indexLifecycleService;
    @Inject
    public TransportExplainLifecycleAction(
        TransportService transportService,