### 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
assertThat(remoteClusterSearchInfo.getSuccessfulShards(), equalTo(remoteNumShards));
            if (minimizeRoundtrips) {
                assertThat(remoteClusterSearchInfo.getSkippedShards(), equalTo(remoteNumShards - 1));
            } else {
                assertThat(remoteClusterSearchInfo.getSkippedShards(), equalTo(remoteNumShards));
            }
            assertThat(remoteClusterSearchInfo.getFailedShards(), equalTo(0));
            assertThat(remoteClusterSearchInfo.getFailures().size(), equalTo(0));
            assertThat(remoteClusterSearchInfo.getTook().millis(), greaterThanOrEqualTo(0L));
        }
    }
    public void testClusterDetailsAfterCCSWithFailuresOnAllShards() throws Exception {
        Map<String, Object> testClusterInfo = setupTwoClusters();
        String localIndex = (String) testClusterInfo.get("local.index");
        String remoteIndex = (String) testClusterInfo.get("remote.index");
        int localNumShards = (Integer) testClusterInfo.get("local.num_shards");
        int remoteNumShards = (Integer) testClusterInfo.get("remote.num_shards");
        boolean skipUnavailable = (Boolean) testClusterInfo.get("remote.skip_unavailable");
        SubmitAsyncSearchRequest request = new SubmitAsyncSearchRequest(localIndex, REMOTE_CLUSTER + ":" + remoteIndex);
        request.setCcsMinimizeRoundtrips(randomBoolean());
        request.setWaitForCompletionTimeout(TimeValue.timeValueMillis(1));
        if (randomBoolean()) {
            request.setBatchedReduceSize(randomIntBetween(2, 256));
        }
        request.setKeepOnCompletion(true);
        boolean dfs = randomBoolean();
        if (dfs) {
            request.getSearchRequest().searchType(SearchType.DFS_QUERY_THEN_FETCH);
        }