### 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
public void testCancellationViaTimeoutWithAllowPartialResultsSetToFalse() throws Exception {
        Map<String, Object> testClusterInfo = setupTwoClusters();
        String localIndex = (String) testClusterInfo.get("local.index");
        String remoteIndex = (String) testClusterInfo.get("remote.index");
        SearchListenerPlugin.blockQueryPhase();
        TimeValue searchTimeout = new TimeValue(100, TimeUnit.MILLISECONDS);
        
        SlowRunningQueryBuilder slowRunningQueryBuilder = new SlowRunningQueryBuilder(searchTimeout.millis() * 5);
        SearchSourceBuilder sourceBuilder = new SearchSourceBuilder().query(slowRunningQueryBuilder).timeout(searchTimeout);
        SubmitAsyncSearchRequest request = new SubmitAsyncSearchRequest(localIndex, REMOTE_CLUSTER + ":" + remoteIndex);
        request.setCcsMinimizeRoundtrips(randomBoolean());
        request.getSearchRequest().source(sourceBuilder);
        if (randomBoolean()) {
            request.setBatchedReduceSize(randomIntBetween(2, 256));
        }
        request.setWaitForCompletionTimeout(TimeValue.timeValueMillis(1));
        request.getSearchRequest().allowPartialSearchResults(false);
        request.setWaitForCompletionTimeout(TimeValue.timeValueMillis(1));
        request.setKeepOnCompletion(true);
        AsyncSearchResponse response = submitAsyncSearch(request);
        assertNotNull(response.getSearchResponse());
        assertTrue(response.isRunning());
        SearchListenerPlugin.waitSearchStarted();
        
        try {
            ListTasksResponse listTasksResponse = client(LOCAL_CLUSTER).admin()
                .cluster()
                .prepareListTasks()
                .setActions(SearchAction.INSTANCE.name())
                .get();