### 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.common.Strings;
import org.elasticsearch.common.settings.Setting;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.xcontent.ChunkedToXContent;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.index.IndexModule;
import org.elasticsearch.index.query.MatchAllQueryBuilder;
import org.elasticsearch.index.query.RangeQueryBuilder;
import org.elasticsearch.index.shard.SearchOperationListener;
import org.elasticsearch.plugins.Plugin;
import org.elasticsearch.plugins.SearchPlugin;
import org.elasticsearch.rest.RestStatus;
import org.elasticsearch.search.builder.SearchSourceBuilder;
import org.elasticsearch.search.internal.LegacyReaderContext;
import org.elasticsearch.search.internal.ReaderContext;
import org.elasticsearch.search.internal.SearchContext;
import org.elasticsearch.search.query.SlowRunningQueryBuilder;
import org.elasticsearch.search.query.ThrowingQueryBuilder;
import org.elasticsearch.tasks.CancellableTask;
import org.elasticsearch.tasks.TaskCancelledException;
import org.elasticsearch.tasks.TaskInfo;
import org.elasticsearch.test.AbstractMultiClustersTestCase;
import org.elasticsearch.test.InternalTestCluster;
import org.elasticsearch.transport.RemoteClusterAware;
import org.elasticsearch.transport.TransportService;
import org.elasticsearch.xcontent.ToXContent;
import org.elasticsearch.xcontent.XContentFactory;
import org.elasticsearch.xpack.async.AsyncResultsIndexPlugin;
import org.elasticsearch.xpack.core.LocalStateCompositeXPackPlugin;
import org.elasticsearch.xpack.core.async.DeleteAsyncResultAction;