### 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
searchTransportService,
                        connectionLookup,
                        aliasFilter,
                        concreteIndexBoosts,
                        executor,
                        queryResultConsumer,
                        searchRequest,
                        listener,
                        shardIterators,
                        timeProvider,
                        clusterState,
                        task,
                        clusters
                    );
                }
            }
        }
    }
    private static void validateAndResolveWaitForCheckpoint(
        ClusterState clusterState,
        IndexNameExpressionResolver resolver,
        SearchRequest searchRequest,
        String[] concreteLocalIndices
    ) {
        HashSet<String> searchedIndices = new HashSet<>(Arrays.asList(concreteLocalIndices));
        Map<String, long[]> newWaitForCheckpoints = Maps.newMapWithExpectedSize(searchRequest.getWaitForCheckpoints().size());
        for (Map.Entry<String, long[]> waitForCheckpointIndex : searchRequest.getWaitForCheckpoints().entrySet()) {
            long[] checkpoints = waitForCheckpointIndex.getValue();
            int checkpointsProvided = checkpoints.length;
            String target = waitForCheckpointIndex.getKey();