### 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
phases.put("warm", new Phase("warm", TimeValue.ZERO, warmActions));
            LifecyclePolicy lifecyclePolicy = new LifecyclePolicy("shrink-only-policy", phases);
            
            XContentBuilder builder = jsonBuilder();
            lifecyclePolicy.toXContent(builder, null);
            final StringEntity entity = new StringEntity("{ \"policy\":" + Strings.toString(builder) + "}", ContentType.APPLICATION_JSON);
            Request request = new Request("PUT", "_ilm/policy/shrink-only-policy");
            request.setEntity(entity);
            assertOK(client().performRequest(request));
        }
        createIndexWithSettings(
            client(),
            goodIndex,
            alias,
            Settings.builder()
                .put(RolloverAction.LIFECYCLE_ROLLOVER_ALIAS, alias)
                .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
                .put(LifecycleSettings.LIFECYCLE_NAME, policy)
        );
        createIndexWithSettings(
            client(),
            errorIndex,
            Settings.builder().put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0).put(LifecycleSettings.LIFECYCLE_NAME, "shrink-only-policy")
        );
        createIndexWithSettings(
            client(),
            nonexistantPolicyIndex,
            Settings.builder()
                .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
                .put(LifecycleSettings.LIFECYCLE_NAME, randomValueOtherThan(policy, () -> randomAlphaOfLengthBetween(3, 10)))