### 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/main/java/org/elasticsearch/action/search/TransportSearchAction.java
final QueryPhaseResultConsumer queryResultConsumer = searchPhaseController.newSearchPhaseResults(
                    executor,
                    circuitBreaker,
                    task::isCancelled,
                    task.getProgressListener(),
                    searchRequest,
                    shardIterators.size(),
                    exc -> searchTransportService.cancelSearchTask(task, "failed to merge result [" + exc.getMessage() + "]")
                );
                if (searchRequest.searchType() == DFS_QUERY_THEN_FETCH) {
                    return new SearchDfsQueryThenFetchAsyncAction(
                        logger,
                        searchTransportService,
                        connectionLookup,
                        aliasFilter,
                        concreteIndexBoosts,
                        executor,
                        queryResultConsumer,
                        searchRequest,
                        listener,
                        shardIterators,
                        timeProvider,
                        clusterState,
                        task,
                        clusters
                    );
                } else {
                    assert searchRequest.searchType() == QUERY_THEN_FETCH : searchRequest.searchType();
                    return new SearchQueryThenFetchAsyncAction(
                        logger,