### Elasticsearch Version

7.x, 8.x

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

Darwin 

### Problem Description

A data stream must have an index template that matches it in order to exist.
When managing index templates we want to make sure we don't leave a
data stream without a [corresponding index template](https://github.com/elastic/elasticsearch/blob/acf9968f9d397d755d5ad9fcf815cca23bbeda61/server/src/main/java/org/elasticsearch/cluster/metadata/MetadataIndexTemplateService.java#L918).

The `dataStreamsUsingTemplate` check uses the index pattern of the index 
templates to conclude if a template is used by a data stream, however it should
be enhanced to check the priority too as there can only be only one index template
that matches a data stream and it's the one with the highest priority.

### Steps to Reproduce

Create an ILM policy
```
PUT _ilm/policy/testing_yo
{
  "policy": {
    "phases": {
      "warm": {
	"actions": {
	  "shrink": {
          "number_of_shards": 1
	  }
        }
      }
    } 
  }
}
```

Create an index template with the `data_stream` option:
```
PUT _index_template/logs_data_stream
{
	"index_patterns": [ "logs*", "te*", "bar*" ],
	"data_stream": {},
        "priority": 100,
	"template" : {
	  "settings": {
            "index.number_of_shards": 3,
	    "index.lifecycle.name": "testing"
	  }
	}
}
```

Create the data stream
```
POST logs_test/_doc 
{
  "message": "testing",
  "@timestamp": "1591890611"
}
```

Create a new index template that would match this data stream with a higher priority
```
PUT _index_template/higher_logs_data_stream
{
	"index_patterns": [ "logs*", "te*", "bar*" ],
	"data_stream": {},
        "priority": 500,
	"template" : {
	  "settings": {
            "index.number_of_shards": 3,
	    "index.lifecycle.name": "testing"
	  }
	}
}
```

Attempt to delete the previous, lower priority index template:
```
DELETE _index_template/logs_data_stream
```

### Logs (if relevant)

_No response_
modules/data-streams/src/internalClusterTest/java/org/elasticsearch/datastreams/DataStreamIT.java
client().execute(CreateDataStreamAction.INSTANCE, createDataStreamRequest).actionGet();
        String backingIndex1 = DataStream.getDefaultBackingIndexName("logs-foobar", 1);
        String backingIndex2 = DataStream.getDefaultBackingIndexName("logs-foobar", 2);
        RolloverResponse rolloverResponse = client().admin().indices().rolloverIndex(new RolloverRequest("logs-foobar", null)).get();
        assertThat(rolloverResponse.getNewIndex(), equalTo(backingIndex2));
        assertTrue(rolloverResponse.isRolledOver());
        
        GetSettingsResponse getSettingsResponse = client().admin().indices().prepareGetSettings("logs-foobar").get();
        assertThat(getSettingsResponse.getIndexToSettings().size(), equalTo(2));
        assertThat(getSettingsResponse.getSetting(backingIndex1, "index.number_of_replicas"), equalTo("1"));
        assertThat(getSettingsResponse.getSetting(backingIndex2, "index.number_of_replicas"), equalTo("1"));
        client().admin()
            .indices()
            .prepareUpdateSettings("logs-foobar")
            .setSettings(Settings.builder().put("index.number_of_replicas", 0))
            .get();
        getSettingsResponse = client().admin().indices().prepareGetSettings("logs-foobar").get();
        assertThat(getSettingsResponse.getIndexToSettings().size(), equalTo(2));
        assertThat(getSettingsResponse.getSetting(backingIndex1, "index.number_of_replicas"), equalTo("0"));
        assertThat(getSettingsResponse.getSetting(backingIndex2, "index.number_of_replicas"), equalTo("0"));
    }
    public void testIndexDocsWithCustomRoutingTargetingDataStreamIsNotAllowed() throws Exception {
        putComposableIndexTemplate("id1", List.of("logs-foo*"));
        
        String dataStream = "logs-foobar";
        IndexRequest indexRequest = new IndexRequest(dataStream).source("{\"@timestamp\": \"2020-12-12\"}", XContentType.JSON)
            .opType(DocWriteRequest.OpType.CREATE);
        IndexResponse indexResponse = client().index(indexRequest).actionGet();
        assertThat(indexResponse.getIndex(), backingIndexEqualTo(dataStream, 1));