### 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/test/java/org/elasticsearch/action/search/SearchResponseTests.java
failedShards = 5;
                failed--;
            } else {
                throw new IllegalStateException("Test setup coding error - should not get here");
            }
            String clusterAlias = "";
            if (i >= 0) {
                clusterAlias = "cluster_" + i;
            }
            AtomicReference<SearchResponse.Cluster> clusterRef = clusters.getCluster(clusterAlias);
            SearchResponse.Cluster cluster = clusterRef.get();
            SearchResponse.Cluster update = new SearchResponse.Cluster(
                cluster.getClusterAlias(),
                cluster.getIndexExpression(),
                false,
                status,
                totalShards,
                successfulShards,
                skippedShards,
                failedShards,
                failureList,
                took,
                false
            );
            assertTrue(clusterRef.compareAndSet(cluster, update));
        }
        return clusters;
    }
    /**
     * the "_shard/total/failures" section makes it impossible to directly