### 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/src/main/java/org/elasticsearch/xpack/ilm/action/TransportExplainLifecycleAction.java
ClusterService clusterService,
        ThreadPool threadPool,
        ActionFilters actionFilters,
        IndexNameExpressionResolver indexNameExpressionResolver,
        NamedXContentRegistry xContentRegistry,
        IndexLifecycleService indexLifecycleService
    ) {
        super(
            ExplainLifecycleAction.NAME,
            transportService,
            clusterService,
            threadPool,
            actionFilters,
            ExplainLifecycleRequest::new,
            indexNameExpressionResolver,
            ExplainLifecycleResponse::new
        );
        this.xContentRegistry = xContentRegistry;
        this.indexLifecycleService = indexLifecycleService;
    }
    @Override
    protected void doMasterOperation(
        Task task,
        ExplainLifecycleRequest request,
        String[] concreteIndices,
        ClusterState state,
        ActionListener<ExplainLifecycleResponse> listener
    ) {
        Map<String, IndexLifecycleExplainResponse> indexResponses = new HashMap<>();
        for (String index : concreteIndices) {