### 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/SearchResponse.java
timedOut,
            terminatedEarly,
            profile,
            numReducePhases
        );
        return new SearchResponse(
            searchResponseSections,
            scrollId,
            totalShards,
            successfulShards,
            skippedShards,
            tookInMillis,
            failures.toArray(ShardSearchFailure.EMPTY_ARRAY),
            clusters,
            searchContextId
        );
    }
    @Override
    public void writeTo(StreamOutput out) throws IOException {
        internalResponse.writeTo(out);
        out.writeVInt(totalShards);
        out.writeVInt(successfulShards);
        out.writeVInt(shardFailures.length);
        for (ShardSearchFailure shardSearchFailure : shardFailures) {
            shardSearchFailure.writeTo(out);
        }
        clusters.writeTo(out);
        out.writeOptionalString(scrollId);
        out.writeVLong(tookInMillis);
        out.writeVInt(skippedShards);