### 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/qa/multi-node/src/javaRestTest/java/org/elasticsearch/xpack/ilm/ExplainLifecycleIT.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.ilm;
import org.apache.http.entity.ContentType;
import org.apache.http.entity.StringEntity;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.elasticsearch.client.Request;
import org.elasticsearch.cluster.metadata.IndexMetadata;