### 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(localClusterSearchInfo.getFailedShards(), equalTo(1));
            assertThat(localClusterSearchInfo.getFailures().size(), equalTo(1));
            assertThat(localClusterSearchInfo.getTook().millis(), greaterThan(0L));
            ShardSearchFailure localShardSearchFailure = localClusterSearchInfo.getFailures().get(0);
            assertTrue("should have 'index corrupted' in reason", localShardSearchFailure.reason().contains("index corrupted"));
            SearchResponse.Cluster remoteClusterSearchInfo = clusters.getCluster(REMOTE_CLUSTER).get();
            assertNotNull(remoteClusterSearchInfo);
            assertThat(remoteClusterSearchInfo.getStatus(), equalTo(SearchResponse.Cluster.Status.PARTIAL));
            assertThat(remoteClusterSearchInfo.getTotalShards(), equalTo(remoteNumShards));
            assertThat(remoteClusterSearchInfo.getSuccessfulShards(), equalTo(remoteNumShards - 1));
            assertThat(remoteClusterSearchInfo.getSkippedShards(), equalTo(0));
            assertThat(remoteClusterSearchInfo.getFailedShards(), equalTo(1));
            assertThat(remoteClusterSearchInfo.getFailures().size(), equalTo(1));
            assertThat(remoteClusterSearchInfo.getTook().millis(), greaterThan(0L));
            ShardSearchFailure remoteShardSearchFailure = remoteClusterSearchInfo.getFailures().get(0);
            assertTrue("should have 'index corrupted' in reason", remoteShardSearchFailure.reason().contains("index corrupted"));
        }
    }
    public void testClusterDetailsAfterCCSWithFailuresOnOneClusterOnly() 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");
        SearchListenerPlugin.blockQueryPhase();
        SubmitAsyncSearchRequest request = new SubmitAsyncSearchRequest(localIndex, REMOTE_CLUSTER + ":" + remoteIndex);
        request.setCcsMinimizeRoundtrips(randomBoolean());
        request.setWaitForCompletionTimeout(TimeValue.timeValueMillis(1));
        request.setKeepOnCompletion(true);