### 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 List<SearchShardIterator> remoteShardIterators = new ArrayList<>();
        for (Map.Entry<String, SearchShardsResponse> entry : searchShardsResponses.entrySet()) {
            for (SearchShardsGroup group : entry.getValue().getGroups()) {
                final ShardId shardId = group.shardId();
                final String clusterAlias = entry.getKey();
                final SearchContextIdForNode perNode = searchContextId.shards().get(shardId);
                assert clusterAlias.equals(perNode.getClusterAlias()) : clusterAlias + " != " + perNode.getClusterAlias();
                final List<String> targetNodes = new ArrayList<>(group.allocatedNodes().size());
                targetNodes.add(perNode.getNode());
                if (perNode.getSearchContextId().getSearcherId() != null) {
                    for (String node : group.allocatedNodes()) {
                        if (node.equals(perNode.getNode()) == false) {
                            targetNodes.add(node);
                        }
                    }
                }
                assert remoteClusterIndices.get(clusterAlias) != null : "original indices are null for clusterAlias: " + clusterAlias;
                final OriginalIndices finalIndices = new OriginalIndices(
                    new String[] { shardId.getIndexName() },
                    remoteClusterIndices.get(clusterAlias).indicesOptions()
                );
                SearchShardIterator shardIterator = new SearchShardIterator(
                    clusterAlias,
                    shardId,
                    targetNodes,
                    finalIndices,
                    perNode.getSearchContextId(),
                    searchContextKeepAlive,
                    false,