### 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
private static ActionListener<SearchResponse> createCCSListener(
        String clusterAlias,
        boolean skipUnavailable,
        CountDown countDown,
        AtomicReference<Exception> exceptions,
        SearchResponseMerger searchResponseMerger,
        SearchResponse.Clusters clusters,
        ActionListener<SearchResponse> originalListener
    ) {
        return new CCSActionListener<SearchResponse, SearchResponse>(
            clusterAlias,
            skipUnavailable,
            countDown,
            exceptions,
            clusters.getCluster(clusterAlias),
            originalListener
        ) {
            @Override
            void innerOnResponse(SearchResponse searchResponse) {
                
                ccsClusterInfoUpdate(searchResponse, cluster, skipUnavailable);
                searchResponseMerger.add(searchResponse);
            }
            @Override
            SearchResponse createFinalResponse() {
                return searchResponseMerger.getMergedResponse(clusters);
            }
        };
    }
    /**