### 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
assertThat(explainIndex.get(IS_AUTO_RETRYABLE_ERROR_FIELD), is(true));
        });
    }
    @SuppressWarnings("unchecked")
    public void testExplainIndicesWildcard() throws Exception {
        createNewSingletonPolicy(client(), policy, "delete", DeleteAction.WITH_SNAPSHOT_DELETE, TimeValue.timeValueDays(100));
        String firstIndex = this.index + "-first";
        String secondIndex = this.index + "-second";
        String unmanagedIndex = this.index + "-unmanaged";
        String indexWithMissingPolicy = this.index + "-missing_policy";
        createIndexWithSettings(
            client(),
            firstIndex,
            alias + firstIndex,
            Settings.builder()
                .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1)
                .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
                .put(LifecycleSettings.LIFECYCLE_NAME, policy)
        );
        createIndexWithSettings(
            client(),
            secondIndex,
            alias + secondIndex,
            Settings.builder()
                .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1)
                .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
                .put(LifecycleSettings.LIFECYCLE_NAME, policy)
        );
        createIndexWithSettings(
            client(),