### 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 
x-pack/plugin/async-search/src/test/java/org/elasticsearch/xpack/search/AsyncSearchResponseTests.java
long tookInMillis = randomNonNegativeLong();
        int totalShards = randomIntBetween(1, Integer.MAX_VALUE);
        int successfulShards = randomIntBetween(0, totalShards);
        int skippedShards = randomIntBetween(0, successfulShards);
        InternalSearchResponse internalSearchResponse = InternalSearchResponse.EMPTY_WITH_TOTAL_HITS;
        SearchResponse.Clusters clusters;
        if (ccs) {
            clusters = createCCSClusterObjects(20, 19, true, 10, 1, 2);
        } else {
            clusters = SearchResponse.Clusters.EMPTY;
        }
        return new SearchResponse(
            internalSearchResponse,
            null,
            totalShards,
            successfulShards,
            skippedShards,
            tookInMillis,
            ShardSearchFailure.EMPTY_ARRAY,
            clusters
        );
    }
    static void assertEqualResponses(AsyncSearchResponse expected, AsyncSearchResponse actual) {
        assertEquals(expected.getId(), actual.getId());
        assertEquals(expected.status(), actual.status());
        assertEquals(expected.getFailure() == null, actual.getFailure() == null);
        assertEquals(expected.isRunning(), actual.isRunning());
        assertEquals(expected.isPartial(), actual.isPartial());
        assertEquals(expected.getStartTime(), actual.getStartTime());
        assertEquals(expected.getExpirationTime(), actual.getExpirationTime());