### 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/internalClusterTest/java/org/elasticsearch/action/search/CCSPointInTimeIT.java
try {
            if (randomBoolean()) {
                localClient.prepareIndex("local_test").setId("local_new").setSource().get();
                localClient.admin().indices().prepareRefresh().get();
            }
            if (randomBoolean()) {
                remoteClient.prepareIndex("remote_test").setId("remote_new").setSource().get();
                remoteClient.admin().indices().prepareRefresh().get();
            }
            
            ThrowingQueryBuilder queryBuilder = new ThrowingQueryBuilder(randomLong(), new IllegalStateException("index corrupted"), 0);
            SearchRequest searchRequest = new SearchRequest();
            searchRequest.source(new SearchSourceBuilder().query(queryBuilder).size(10).pointInTimeBuilder(new PointInTimeBuilder(pitId)));
            SearchResponse searchResponse = client(LOCAL_CLUSTER).search(searchRequest).get();
            SearchResponse.Clusters clusters = searchResponse.getClusters();
            int expectedNumClusters = 1 + (includeLocalIndex ? 1 : 0);
            assertThat(clusters.getTotal(), equalTo(expectedNumClusters));
            assertThat(clusters.getClusterStateCount(SearchResponse.Cluster.Status.SUCCESSFUL), equalTo(0));
            assertThat(clusters.getClusterStateCount(SearchResponse.Cluster.Status.SKIPPED), equalTo(0));
            assertThat(clusters.getClusterStateCount(SearchResponse.Cluster.Status.PARTIAL), equalTo(expectedNumClusters));
            if (includeLocalIndex) {
                AtomicReference<SearchResponse.Cluster> localClusterRef = clusters.getCluster(RemoteClusterAware.LOCAL_CLUSTER_GROUP_KEY);
                assertNotNull(localClusterRef);
                assertOneFailedShard(localClusterRef.get(), numShards);
            }
            AtomicReference<SearchResponse.Cluster> remoteClusterRef = clusters.getCluster(REMOTE_CLUSTER);
            assertNotNull(remoteClusterRef);
            assertOneFailedShard(remoteClusterRef.get(), numShards);
        } finally {
            closePointInTime(pitId);