### 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
private static final NamedXContentRegistry xContentRegistry;
    static {
        List<NamedXContentRegistry.Entry> namedXContents = new ArrayList<>(InternalAggregationTestCase.getDefaultNamedXContents());
        namedXContents.addAll(SuggestTests.getDefaultNamedXContents());
        xContentRegistry = new NamedXContentRegistry(namedXContents);
    }
    private final NamedWriteableRegistry namedWriteableRegistry = new NamedWriteableRegistry(
        new SearchModule(Settings.EMPTY, emptyList()).getNamedWriteables()
    );
    private AggregationsTests aggregationsTests = new AggregationsTests();
    @Before
    public void init() throws Exception {
        aggregationsTests.init();
    }