### 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
SearchModule searchModule = new SearchModule(Settings.EMPTY, emptyList());
        List<NamedWriteableRegistry.Entry> namedWriteables = searchModule.getNamedWriteables();
        namedWriteableRegistry = new NamedWriteableRegistry(namedWriteables);
    }
    protected Writeable.Reader<AsyncSearchResponse> instanceReader() {
        return AsyncSearchResponse::new;
    }
    protected AsyncSearchResponse createTestInstance() {
        return randomAsyncSearchResponse(randomSearchId(), searchResponse);
    }
    protected void assertEqualInstances(AsyncSearchResponse expectedInstance, AsyncSearchResponse newInstance) {
        assertNotSame(newInstance, expectedInstance);
        assertEqualResponses(expectedInstance, newInstance);
    }
    public final void testSerialization() throws IOException {
        for (int runs = 0; runs < 10; runs++) {
            AsyncSearchResponse testInstance = createTestInstance();
            assertSerialization(testInstance);
        }
    }
    protected final AsyncSearchResponse assertSerialization(AsyncSearchResponse testInstance) throws IOException {
        return assertSerialization(testInstance, TransportVersion.current());
    }
    protected final AsyncSearchResponse assertSerialization(AsyncSearchResponse testInstance, TransportVersion version) throws IOException {
        AsyncSearchResponse deserializedInstance = copyInstance(testInstance, version);
        assertEqualInstances(testInstance, deserializedInstance);
        return deserializedInstance;
    }
    protected final AsyncSearchResponse copyInstance(AsyncSearchResponse instance) throws IOException {
        return copyInstance(instance, TransportVersion.current());