### 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/action/search/CCSPointInTimeIT.java
import org.elasticsearch.plugins.Plugin;
import org.elasticsearch.plugins.SearchPlugin;
import org.elasticsearch.search.builder.PointInTimeBuilder;
import org.elasticsearch.search.builder.SearchSourceBuilder;
import org.elasticsearch.search.ccs.CrossClusterSearchIT;
import org.elasticsearch.search.query.ThrowingQueryBuilder;
import org.elasticsearch.test.AbstractMultiClustersTestCase;
import org.elasticsearch.transport.RemoteClusterAware;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.atomic.AtomicReference;
import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertHitCount;
import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertNoFailures;
import static org.hamcrest.Matchers.containsString;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.greaterThanOrEqualTo;
public class CCSPointInTimeIT extends AbstractMultiClustersTestCase {
    public static final String REMOTE_CLUSTER = "remote_cluster";
    @Override
    protected Collection<String> remoteClusterAlias() {
        return List.of(REMOTE_CLUSTER);
    }
    @Override
    protected Collection<Class<? extends Plugin>> nodePlugins(String clusterAlias) {
        return CollectionUtils.appendToCopy(super.nodePlugins(clusterAlias), CrossClusterSearchIT.TestQueryBuilderPlugin.class);
    }
    public static class TestQueryBuilderPlugin extends Plugin implements SearchPlugin {