### Description

`SearchResponse.Clusters` currently uses a `HashMap<String, AtomicReference>` for storing Cluster objects. The AtomicReference CAS swaps are done in a do/while loop in most cases (e.g., in `CCSSingleCoordinatorSearchProgressListener`).

This can be replaced with a `ConcurrentHashMap` in conjunction with either `compute()` method or the `replace(key, assumed_previous_value, value)` method for atomically swapping in a new Cluster object with a CAS operation, assuring that the updates don't lose data from competing threads doing separate Cluster object updates.

See: https://github.com/elastic/elasticsearch/pull/98457/files#r1310467586 
x-pack/plugin/async-search/src/test/java/org/elasticsearch/xpack/search/AsyncSearchResponseTests.java
import org.elasticsearch.index.shard.ShardId;
import org.elasticsearch.script.ScriptException;
import org.elasticsearch.search.SearchHits;
import org.elasticsearch.search.SearchModule;
import org.elasticsearch.search.SearchShardTarget;
import org.elasticsearch.search.internal.InternalSearchResponse;
import org.elasticsearch.test.ESTestCase;
import org.elasticsearch.transport.RemoteClusterAware;
import org.elasticsearch.xcontent.ToXContent;
import org.elasticsearch.xcontent.XContentBuilder;
import org.elasticsearch.xcontent.XContentType;
import org.elasticsearch.xpack.core.search.action.AsyncSearchResponse;
import org.junit.Before;
import java.io.IOException;
import java.time.Instant;
import java.util.Arrays;
import java.util.Collections;
import java.util.Date;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.UUID;
import java.util.concurrent.atomic.AtomicReference;
import static java.util.Collections.emptyList;
import static org.elasticsearch.xpack.core.async.GetAsyncResultRequestTests.randomSearchId;
public class AsyncSearchResponseTests extends ESTestCase {
    private final SearchResponse searchResponse = randomSearchResponse(randomBoolean());
    private NamedWriteableRegistry namedWriteableRegistry;
    @Before
    public void registerNamedObjects() {