### 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/internalClusterTest/java/org/elasticsearch/search/ccs/CrossClusterSearchIT.java
.isTimedOut()
        );
        indexDocs(client(REMOTE_CLUSTER), remoteIndex);
        String skipUnavailableKey = Strings.format("cluster.remote.%s.skip_unavailable", REMOTE_CLUSTER);
        Setting<?> skipUnavailableSetting = cluster(REMOTE_CLUSTER).clusterService().getClusterSettings().get(skipUnavailableKey);
        boolean skipUnavailable = (boolean) cluster(RemoteClusterAware.LOCAL_CLUSTER_GROUP_KEY).clusterService()
            .getClusterSettings()
            .get(skipUnavailableSetting);
        Map<String, Object> clusterInfo = new HashMap<>();
        clusterInfo.put("local.num_shards", numShardsLocal);
        clusterInfo.put("local.index", localIndex);
        clusterInfo.put("remote.num_shards", numShardsRemote);
        clusterInfo.put("remote.index", remoteIndex);
        clusterInfo.put("remote.skip_unavailable", skipUnavailable);
        return clusterInfo;
    }
    private int indexDocs(Client client, String index) {
        int numDocs = between(500, 1200);
        for (int i = 0; i < numDocs; i++) {
            long ts = EARLIEST_TIMESTAMP + i;
            if (i == numDocs - 1) {
                ts = LATEST_TIMESTAMP;
            }
            client.prepareIndex(index).setSource("f", "v", "@timestamp", ts).get();
        }
        client.admin().indices().prepareRefresh(index).get();
        return numDocs;
    }
}