### 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
public TestQueryBuilderPlugin() {}
        @Override
        public List<QuerySpec<?>> getQueries() {
            QuerySpec<ThrowingQueryBuilder> throwingSpec = new QuerySpec<>(ThrowingQueryBuilder.NAME, ThrowingQueryBuilder::new, p -> {
                throw new IllegalStateException("not implemented");
            });
            return List.of(throwingSpec);
        }
    }
    void indexDocs(Client client, String index, int numDocs) {
        for (int i = 0; i < numDocs; i++) {
            String id = Integer.toString(i);
            client.prepareIndex(index).setId(id).setSource("value", i).get();
        }
        client.admin().indices().prepareRefresh(index).get();
    }
    public void testBasic() {
        final Client localClient = client(LOCAL_CLUSTER);
        final Client remoteClient = client(REMOTE_CLUSTER);
        int localNumDocs = randomIntBetween(10, 50);
        assertAcked(localClient.admin().indices().prepareCreate("local_test"));
        indexDocs(localClient, "local_test", localNumDocs);
        int remoteNumDocs = randomIntBetween(10, 50);
        assertAcked(remoteClient.admin().indices().prepareCreate("remote_test"));