### 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/SearchResponse.java
static final ParseField INDICES_FIELD = new ParseField("indices");
        static final ParseField STATUS_FIELD = new ParseField("status");
        private static final boolean SKIP_UNAVAILABLE_DEFAULT = false;
        private final String clusterAlias;
        private final String indexExpression; 
        private final boolean skipUnavailable;
        private final Status status;
        private final Integer totalShards;
        private final Integer successfulShards;
        private final Integer skippedShards;
        private final Integer failedShards;
        private final List<ShardSearchFailure> failures;
        private final TimeValue took;  
        private final boolean timedOut;
        /**
         * Marks the status of a Cluster search involved in a Cross-Cluster search.
         */
        public enum Status {
            RUNNING,     
            SUCCESSFUL,  
            PARTIAL,     
            SKIPPED,     
            FAILED;      
            @Override
            public String toString() {
                return this.name().toLowerCase(Locale.ROOT);
            }
        }
        /**