### 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 
server/src/internalClusterTest/java/org/elasticsearch/search/ccs/CrossClusterSearchIT.java
import org.elasticsearch.client.internal.Client;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.settings.Setting;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.util.CollectionUtils;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.index.query.MatchAllQueryBuilder;
import org.elasticsearch.index.query.RangeQueryBuilder;
import org.elasticsearch.plugins.Plugin;
import org.elasticsearch.plugins.SearchPlugin;
import org.elasticsearch.search.builder.SearchSourceBuilder;
import org.elasticsearch.search.query.SlowRunningQueryBuilder;
import org.elasticsearch.search.query.ThrowingQueryBuilder;
import org.elasticsearch.test.AbstractMultiClustersTestCase;
import org.elasticsearch.test.InternalTestCluster;
import org.elasticsearch.transport.RemoteClusterAware;
import org.elasticsearch.transport.RemoteTransportException;
import java.util.Collection;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;
import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
import static org.hamcrest.Matchers.containsString;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.greaterThan;
import static org.hamcrest.Matchers.greaterThanOrEqualTo;
import static org.hamcrest.Matchers.instanceOf;
public class CrossClusterSearchIT extends AbstractMultiClustersTestCase {