### 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 
x-pack/plugin/async-search/src/test/java/org/elasticsearch/xpack/search/AsyncSearchResponseTests.java
3,
            0,
            Collections.emptyList(),
            new TimeValue(11111),
            false
        );
        boolean swapped = clusterRef.compareAndSet(localCluster, updated);
        assertTrue("CAS swap failed for cluster " + updated, swapped);
        clusterRef = clusters.getCluster("cluster_0");
        SearchResponse.Cluster cluster0 = clusterRef.get();
        updated = new SearchResponse.Cluster(
            cluster0.getClusterAlias(),
            cluster0.getIndexExpression(),
            false,
            SearchResponse.Cluster.Status.SUCCESSFUL,
            8,
            8,
            1,
            0,
            Collections.emptyList(),
            new TimeValue(7777),
            false
        );
        swapped = clusterRef.compareAndSet(cluster0, updated);
        assertTrue("CAS swap failed for cluster " + updated, swapped);
        clusterRef = clusters.getCluster("cluster_1");
        SearchResponse.Cluster cluster1 = clusterRef.get();
        ShardSearchFailure failure1 = new ShardSearchFailure(
            new NullPointerException("NPE details"),
            new SearchShardTarget("nodeId0", new ShardId("foo", UUID.randomUUID().toString(), 0), "cluster_1")