### 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/TransportSearchAction.java
+ ". This limit exists because querying many shards at the same time can make the "
                    + "job of the coordinating node very CPU and/or memory intensive. It is usually a better idea to "
                    + "have a smaller number of larger shards. Update ["
                    + SHARD_COUNT_LIMIT_SETTING.getKey()
                    + "] to a greater value if you really want to query that many shards at the same time."
            );
        }
    }
    abstract static class CCSActionListener<Response, FinalResponse> implements ActionListener<Response> {
        protected final String clusterAlias;
        protected final boolean skipUnavailable;
        private final CountDown countDown;
        private final AtomicReference<Exception> exceptions;
        protected final AtomicReference<SearchResponse.Cluster> cluster;
        private final ActionListener<FinalResponse> originalListener;
        protected final long startTime;
        /**
         * Used by both minimize_roundtrips true and false
         */
        CCSActionListener(
            String clusterAlias,
            boolean skipUnavailable,
            CountDown countDown,
            AtomicReference<Exception> exceptions,
            @Nullable AtomicReference<SearchResponse.Cluster> cluster, 
            ActionListener<FinalResponse> originalListener
        ) {
            this.clusterAlias = clusterAlias;
            this.skipUnavailable = skipUnavailable;
            this.countDown = countDown;