### 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
localShardIterators = getLocalLocalShardsIteratorFromPointInTime(
                clusterState,
                localIndices,
                searchRequest.getLocalClusterAlias(),
                searchContext,
                searchRequest.pointInTimeBuilder().getKeepAlive(),
                searchRequest.allowPartialSearchResults()
            );
        } else {
            final Index[] indices = resolveLocalIndices(localIndices, clusterState, timeProvider);
            concreteLocalIndices = Arrays.stream(indices).map(Index::getName).toArray(String[]::new);
            final Set<String> indicesAndAliases = indexNameExpressionResolver.resolveExpressions(clusterState, searchRequest.indices());
            aliasFilter = buildIndexAliasFilters(clusterState, indicesAndAliases, indices);
            aliasFilter.putAll(remoteAliasMap);
            localShardIterators = getLocalShardsIterator(
                clusterState,
                searchRequest,
                searchRequest.getLocalClusterAlias(),
                indicesAndAliases,
                concreteLocalIndices
            );
        }
        final GroupShardsIterator<SearchShardIterator> shardIterators = mergeShardsIterators(localShardIterators, remoteShardIterators);
        failIfOverShardCountLimit(clusterService, shardIterators.size());
        if (searchRequest.getWaitForCheckpoints().isEmpty() == false) {
            if (remoteShardIterators.isEmpty() == false) {
                throw new IllegalArgumentException("Cannot use wait_for_checkpoints parameter with cross-cluster searches.");
            } else {
                validateAndResolveWaitForCheckpoint(clusterState, indexNameExpressionResolver, searchRequest, concreteLocalIndices);
            }