### 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/search/ccs/CrossClusterSearchIT.java
private static final String REMOTE_CLUSTER = "cluster_a";
    private static long EARLIEST_TIMESTAMP = 1691348810000L;
    private static long LATEST_TIMESTAMP = 1691348820000L;
    @Override
    protected Collection<String> remoteClusterAlias() {
        return List.of(REMOTE_CLUSTER);
    }
    @Override
    protected Map<String, Boolean> skipUnavailableForRemoteClusters() {
        return Map.of(REMOTE_CLUSTER, randomBoolean());
    }
    @Override
    protected boolean reuseClusters() {
        return false;
    }
    @Override
    protected Collection<Class<? extends Plugin>> nodePlugins(String clusterAlias) {
        return CollectionUtils.appendToCopy(super.nodePlugins(clusterAlias), CrossClusterSearchIT.TestQueryBuilderPlugin.class);
    }
    public static class TestQueryBuilderPlugin extends Plugin implements SearchPlugin {