### 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
});
    }
    private void assertUnmanagedIndex(Map<String, Object> explainIndexMap) {
        assertThat(explainIndexMap.get("managed"), is(false));
        assertThat(explainIndexMap.get("time_since_index_creation"), is(nullValue()));
        assertThat(explainIndexMap.get("index_creation_date_millis"), is(nullValue()));
        assertThat(explainIndexMap.get("policy"), is(nullValue()));
        assertThat(explainIndexMap.get("phase"), is(nullValue()));
        assertThat(explainIndexMap.get("action"), is(nullValue()));
        assertThat(explainIndexMap.get("step"), is(nullValue()));
        assertThat(explainIndexMap.get("age"), is(nullValue()));
        assertThat(explainIndexMap.get("failed_step"), is(nullValue()));
        assertThat(explainIndexMap.get("step_info"), is(nullValue()));
    }
    private void assertManagedIndex(Map<String, Object> explainIndexMap) {
        assertThat(explainIndexMap.get("managed"), is(true));
        assertThat(explainIndexMap.get("time_since_index_creation"), is(notNullValue()));
        assertThat(explainIndexMap.get("index_creation_date_millis"), is(notNullValue()));
        assertThat(explainIndexMap.get("policy"), is(policy));
        assertThat(explainIndexMap.get("phase"), is("new"));
        assertThat(explainIndexMap.get("action"), is("complete"));
        assertThat(explainIndexMap.get("step"), is("complete"));
        assertThat(explainIndexMap.get("phase_time_millis"), is(notNullValue()));
        assertThat(explainIndexMap.get("age"), is(notNullValue()));
        assertThat(explainIndexMap.get("phase_execution"), is(notNullValue()));
        assertThat(explainIndexMap.get("failed_step"), is(nullValue()));
        assertThat(explainIndexMap.get("step_info"), is(nullValue()));
    }
}