### 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/main/java/org/elasticsearch/action/search/CCSSingleCoordinatorSearchProgressListener.java
private SearchResponse.Clusters clusters;
    private TransportSearchAction.SearchTimeProvider timeProvider;
    /**
     * Executed when shards are ready to be queried (after can-match)
     *
     * @param shards The list of shards to query.
     * @param skipped The list of skipped shards.
     * @param clusters The statistics for remote clusters included in the search.
     * @param fetchPhase <code>true</code> if the search needs a fetch phase, <code>false</code> otherwise.
     **/
    @Override
    public void onListShards(
        List<SearchShard> shards,
        List<SearchShard> skipped,
        SearchResponse.Clusters clusters,
        boolean fetchPhase,
        TransportSearchAction.SearchTimeProvider timeProvider
    ) {
        assert clusters.isCcsMinimizeRoundtrips() == false : "minimize_roundtrips must be false to use this SearchListener";