### 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
Index resolved;
            try {
                resolved = resolver.concreteSingleIndex(clusterState, new IndicesRequest() {
                    @Override
                    public String[] indices() {
                        return new String[] { target };
                    }
                    @Override
                    public IndicesOptions indicesOptions() {
                        return IndicesOptions.strictSingleIndexNoExpandForbidClosed();
                    }
                });
            } catch (Exception e) {
                throw new IllegalArgumentException(
                    "Failed to resolve wait_for_checkpoints target ["
                        + target
                        + "]. Configured target "
                        + "must resolve to a single open index.",
                    e
                );
            }
            String index = resolved.getName();
            IndexMetadata indexMetadata = clusterState.metadata().index(index);
            if (searchedIndices.contains(index) == false) {
                throw new IllegalArgumentException(
                    "Target configured with wait_for_checkpoints must be a concrete index resolved in "
                        + "this search. Target ["
                        + target
                        + "] is not a concrete index resolved in this search."
                );