### 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
if (includeLocalIndex) {
                AtomicReference<SearchResponse.Cluster> localClusterRef = clusters.getCluster(RemoteClusterAware.LOCAL_CLUSTER_GROUP_KEY);
                assertNotNull(localClusterRef);
                assertOneSuccessfulShard(localClusterRef.get());
            }
            AtomicReference<SearchResponse.Cluster> remoteClusterRef = clusters.getCluster(REMOTE_CLUSTER);
            assertNotNull(remoteClusterRef);
            assertOneSuccessfulShard(remoteClusterRef.get());
        } finally {
            closePointInTime(pitId);
        }
    }
    public void testFailuresOnOneShardsWithPointInTime() throws ExecutionException, InterruptedException {
        final Client localClient = client(LOCAL_CLUSTER);
        final Client remoteClient = client(REMOTE_CLUSTER);
        int localNumDocs = randomIntBetween(10, 50);
        int numShards = randomIntBetween(2, 4);
        Settings clusterSettings = indexSettings(numShards, randomIntBetween(0, 1)).build();
        assertAcked(localClient.admin().indices().prepareCreate("local_test").setSettings(clusterSettings));
        indexDocs(localClient, "local_test", localNumDocs);
        int remoteNumDocs = randomIntBetween(10, 50);
        assertAcked(remoteClient.admin().indices().prepareCreate("remote_test").setSettings(clusterSettings));
        indexDocs(remoteClient, "remote_test", remoteNumDocs);
        boolean includeLocalIndex = randomBoolean();
        List<String> indices = new ArrayList<>();
        if (includeLocalIndex) {
            indices.add(randomFrom("*", "local_*", "local_test"));
        }
        indices.add(randomFrom("*:*", "remote_cluster:*", "remote_cluster:remote_test"));
        String pitId = openPointInTime(indices.toArray(new String[0]), TimeValue.timeValueMinutes(2));