### 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
task,
                    true,
                    searchService.getCoordinatorRewriteContextProvider(timeProvider::absoluteStartMillis),
                    listener.delegateFailureAndWrap((l, iters) -> {
                        SearchPhase action = newSearchPhase(
                            task,
                            searchRequest,
                            executor,
                            iters,
                            timeProvider,
                            connectionLookup,
                            clusterState,
                            aliasFilter,
                            concreteIndexBoosts,
                            false,
                            threadPool,
                            clusters
                        );
                        action.start();
                    })
                );
            } else {
                
                
                
                if (clusters.isCcsMinimizeRoundtrips() == false
                    && clusters.hasRemoteClusters()
                    && task.getProgressListener() == SearchProgressListener.NOOP) {
                    task.setProgressListener(new CCSSingleCoordinatorSearchProgressListener());
                }