### 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
unmanagedIndex,
            alias + unmanagedIndex,
            Settings.builder().put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1).put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
        );
        String missingPolicyName = "missing_policy_";
        createIndexWithSettings(
            client(),
            indexWithMissingPolicy,
            alias + indexWithMissingPolicy,
            Settings.builder()
                .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1)
                .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
                .put(LifecycleSettings.LIFECYCLE_NAME, missingPolicyName)
        );
        assertBusy(() -> {
            Map<String, Map<String, Object>> explain = explain(client(), this.index + "*", false, false);
            assertManagedIndex(explain.get(firstIndex));
            assertManagedIndex(explain.get(secondIndex));
            assertUnmanagedIndex(explain.get(unmanagedIndex));
            Map<String, Object> explainIndexWithMissingPolicy = explain.get(indexWithMissingPolicy);
            assertThat(explainIndexWithMissingPolicy.get("managed"), is(true));
            assertThat(explainIndexWithMissingPolicy.get("policy"), is(missingPolicyName));
            assertThat(explainIndexWithMissingPolicy.get("phase"), is(nullValue()));
            assertThat(explainIndexWithMissingPolicy.get("action"), is(nullValue()));
            assertThat(explainIndexWithMissingPolicy.get("step"), is(nullValue()));
            assertThat(explainIndexWithMissingPolicy.get("age"), is(nullValue()));
            assertThat(explainIndexWithMissingPolicy.get("failed_step"), is(nullValue()));
            Map<String, Object> stepInfo = (Map<String, Object>) explainIndexWithMissingPolicy.get("step_info");
            assertThat(stepInfo, is(notNullValue()));
            assertThat(stepInfo.get("reason"), is("policy [missing_policy_] does not exist"));