### 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
"total": {
                      "value": 100,
                      "relation": "eq"
                    },
                    "max_score": 1.5,
                    "hits": [
                      {
                        "_id": "id1",
                        "_score": 2.0
                      }
                    ]
                  }
                }""");
            assertEquals(expectedString, Strings.toString(response));
        }
    }
    public void testSerialization() throws IOException {
        SearchResponse searchResponse = createTestItem(false);
        SearchResponse deserialized = copyWriteable(
            searchResponse,
            namedWriteableRegistry,
            SearchResponse::new,
            TransportVersion.current()
        );
        if (searchResponse.getHits().getTotalHits() == null) {
            assertNull(deserialized.getHits().getTotalHits());
        } else {
            assertEquals(searchResponse.getHits().getTotalHits().value, deserialized.getHits().getTotalHits().value);
            assertEquals(searchResponse.getHits().getTotalHits().relation, deserialized.getHits().getTotalHits().relation);
        }