### 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/internalClusterTest/java/org/elasticsearch/xpack/search/CrossClusterAsyncSearchIT.java
if (randomBoolean()) {
            request.setBatchedReduceSize(randomIntBetween(2, 256));
        }
        boolean dfs = randomBoolean();
        if (dfs) {
            request.getSearchRequest().searchType(SearchType.DFS_QUERY_THEN_FETCH);
        }
        
        ThrowingQueryBuilder queryBuilder = new ThrowingQueryBuilder(
            randomLong(),
            new IllegalStateException("index corrupted"),
            remoteIndex
        );
        request.getSearchRequest().source(new SearchSourceBuilder().query(queryBuilder).size(10));
        boolean minimizeRoundtrips = TransportSearchAction.shouldMinimizeRoundtrips(request.getSearchRequest());
        AsyncSearchResponse response = submitAsyncSearch(request);
        assertNotNull(response.getSearchResponse());
        assertTrue(response.isRunning());
        {
            SearchResponse.Clusters clusters = response.getSearchResponse().getClusters();
            assertThat(clusters.getTotal(), equalTo(2));
            assertTrue("search cluster results should be marked as partial", clusters.hasPartialResults());
            SearchResponse.Cluster localClusterSearchInfo = clusters.getCluster(RemoteClusterAware.LOCAL_CLUSTER_GROUP_KEY).get();
            assertNotNull(localClusterSearchInfo);
            assertThat(localClusterSearchInfo.getStatus(), equalTo(SearchResponse.Cluster.Status.RUNNING));
            SearchResponse.Cluster remoteClusterSearchInfo = clusters.getCluster(REMOTE_CLUSTER).get();
            assertNotNull(remoteClusterSearchInfo);
            assertThat(localClusterSearchInfo.getStatus(), equalTo(SearchResponse.Cluster.Status.RUNNING));
        }
        SearchListenerPlugin.waitSearchStarted();