### 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
* compare xContent, so we omit it here
     */
    public void testFromXContent() throws IOException {
        doFromXContentTestWithRandomFields(createTestItem(), false);
    }
    /**
     * This test adds random fields and objects to the xContent rendered out to
     * ensure we can parse it back to be forward compatible with additions to
     * the xContent. We test this with a "minimal" SearchResponse, adding random
     * fields to SearchHits, Aggregations etc... is tested in their own tests
     */
    public void testFromXContentWithRandomFields() throws IOException {
        doFromXContentTestWithRandomFields(createMinimalTestItem(), true);
    }
    private void doFromXContentTestWithRandomFields(SearchResponse response, boolean addRandomFields) throws IOException {
        XContentType xcontentType = randomFrom(XContentType.values());
        boolean humanReadable = randomBoolean();
        final ToXContent.Params params = new ToXContent.MapParams(singletonMap(RestSearchAction.TYPED_KEYS_PARAM, "true"));
        BytesReference originalBytes = toShuffledXContent(
            ChunkedToXContent.wrapAsToXContent(response),
            xcontentType,
            params,
            humanReadable
        );
        BytesReference mutated;
        if (addRandomFields) {
            mutated = insertRandomFields(xcontentType, originalBytes, null, random());
        } else {
            mutated = originalBytes;
        }