### 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/internalClusterTest/java/org/elasticsearch/xpack/search/CrossClusterAsyncSearchIT.java
import org.elasticsearch.xpack.core.async.DeleteAsyncResultRequest;
import org.elasticsearch.xpack.core.async.GetAsyncResultRequest;
import org.elasticsearch.xpack.core.async.GetAsyncStatusRequest;
import org.elasticsearch.xpack.core.search.action.AsyncSearchResponse;
import org.elasticsearch.xpack.core.search.action.AsyncStatusResponse;
import org.elasticsearch.xpack.core.search.action.GetAsyncSearchAction;
import org.elasticsearch.xpack.core.search.action.GetAsyncStatusAction;
import org.elasticsearch.xpack.core.search.action.SubmitAsyncSearchAction;
import org.elasticsearch.xpack.core.search.action.SubmitAsyncSearchRequest;
import org.hamcrest.Matchers;
import org.junit.Before;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.greaterThan;
import static org.hamcrest.Matchers.greaterThanOrEqualTo;
import static org.hamcrest.Matchers.instanceOf;
import static org.hamcrest.Matchers.matchesRegex;
import static org.hamcrest.Matchers.not;
public class CrossClusterAsyncSearchIT extends AbstractMultiClustersTestCase {