### 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
"successful": 0,
                    "skipped": 0,
                    "failed": 0
                  },
                  "_clusters": {
                    "total": 5,
                    "successful": 3,
                    "skipped": 2,
                    "running":0,
                    "partial": 0,
                    "failed": 0
                  },
                  "hits": {
                    "total": {
                      "value": 100,
                      "relation": "eq"
                    },
                    "max_score": 1.5,
                    "hits": [ { "_id": "id1", "_score": 2.0 } ]
                  }
                }""");
            assertEquals(expectedString, Strings.toString(response));
        }
        {
            SearchResponse response = new SearchResponse(
                new InternalSearchResponse(
                    new SearchHits(hits, new TotalHits(100, TotalHits.Relation.EQUAL_TO), 1.5f),
                    null,
                    null,
                    null,