### 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
0,
            Collections.emptyList(),
            new TimeValue(17322),
            true
        );
        swapped = clusterRef.compareAndSet(cluster2, updated);
        assertTrue("CAS swap failed for cluster " + updated, swapped);
        SearchResponse searchResponse = new SearchResponse(sections, null, 10, 9, 1, took, new ShardSearchFailure[0], clusters);
        AsyncSearchResponse asyncSearchResponse = new AsyncSearchResponse(
            "id",
            searchResponse,
            null,
            false,
            isRunning,
            startTimeMillis,
            expirationTimeMillis
        );
        try (XContentBuilder builder = XContentBuilder.builder(XContentType.JSON.xContent())) {
            builder.prettyPrint();
            ChunkedToXContent.wrapAsToXContent(asyncSearchResponse).toXContent(builder, ToXContent.EMPTY_PARAMS);
            assertEquals(Strings.format("""
                {
                  "id" : "id",
                  "is_partial" : false,
                  "is_running" : false,
                  "start_time_in_millis" : %s,
                  "expiration_time_in_millis" : %s,
                  "completion_time_in_millis" : %s,
                  "response" : {
                    "took" : %s,