### 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
BulkResponse bulkResponse = client().bulk(bulkRequest).actionGet();
        assertThat(bulkResponse.getItems().length, equalTo(numDocs));
        String backingIndexPrefix = DataStream.BACKING_INDEX_PREFIX + dataStream;
        for (BulkItemResponse itemResponse : bulkResponse) {
            assertThat(itemResponse.getFailureMessage(), nullValue());
            assertThat(itemResponse.status(), equalTo(RestStatus.CREATED));
            assertThat(itemResponse.getIndex(), startsWith(backingIndexPrefix));
        }
        client().admin().indices().refresh(new RefreshRequest(dataStream)).actionGet();
    }
    static void verifyDocs(String dataStream, long expectedNumHits, List<String> expectedIndices) {
        SearchRequest searchRequest = new SearchRequest(dataStream);
        searchRequest.source().size((int) expectedNumHits);
        SearchResponse searchResponse = client().search(searchRequest).actionGet();
        assertThat(searchResponse.getHits().getTotalHits().value, equalTo(expectedNumHits));
        Arrays.stream(searchResponse.getHits().getHits()).forEach(hit -> { assertTrue(expectedIndices.contains(hit.getIndex())); });
    }
    static void verifyDocs(String dataStream, long expectedNumHits, long minGeneration, long maxGeneration) {
        List<String> expectedIndices = new ArrayList<>();
        for (long k = minGeneration; k <= maxGeneration; k++) {
            expectedIndices.add(DataStream.getDefaultBackingIndexName(dataStream, k));
        }
        verifyDocs(dataStream, expectedNumHits, expectedIndices);
    }
    public static void putComposableIndexTemplate(String id, List<String> patterns) throws IOException {
        putComposableIndexTemplate(id, null, patterns, null, null);
    }
    public void testPartitionedTemplate() throws IOException {
        /**
         * partition size with no routing required