### 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/test/java/org/elasticsearch/action/search/SearchResponseTests.java
if (minimal == false) {
            SearchHits hits = SearchHitsTests.createTestItem(true, true);
            InternalAggregations aggregations = aggregationsTests.createTestInstance();
            Suggest suggest = SuggestTests.createTestItem();
            SearchProfileResults profileResults = SearchProfileResultsTests.createTestItem();
            internalSearchResponse = new InternalSearchResponse(
                hits,
                aggregations,
                suggest,
                profileResults,
                timedOut,
                terminatedEarly,
                numReducePhases
            );
        } else {
            internalSearchResponse = InternalSearchResponse.EMPTY_WITH_TOTAL_HITS;
        }
        SearchResponse.Clusters clusters;
        if (minimal) {
            clusters = randomSimpleClusters();
        } else {
            clusters = randomClusters();
        }
        return new SearchResponse(
            internalSearchResponse,
            null,
            totalShards,
            successfulShards,
            skippedShards,
            tookInMillis,