### 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
+ skipUnavailable
                + '}';
        }
    }
    
    public static SearchResponse empty(Supplier<Long> tookInMillisSupplier, Clusters clusters) {
        SearchHits searchHits = new SearchHits(new SearchHit[0], new TotalHits(0L, TotalHits.Relation.EQUAL_TO), Float.NaN);
        InternalSearchResponse internalSearchResponse = new InternalSearchResponse(
            searchHits,
            InternalAggregations.EMPTY,
            null,
            null,
            false,
            null,
            0
        );
        return new SearchResponse(
            internalSearchResponse,
            null,
            0,
            0,
            0,
            tookInMillisSupplier.get(),
            ShardSearchFailure.EMPTY_ARRAY,
            clusters,
            null
        );
    }
}