### 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/internalClusterTest/java/org/elasticsearch/xpack/search/CrossClusterAsyncSearchIT.java
public void onNewReaderContext(ReaderContext readerContext) {
                    assertThat(readerContext, not(instanceOf(LegacyReaderContext.class)));
                }
                @Override
                public void onPreQueryPhase(SearchContext searchContext) {
                    startedLatch.get().countDown();
                    final CountDownLatch latch = queryLatch.get();
                    if (latch != null) {
                        try {
                            assertTrue(latch.await(60, TimeUnit.SECONDS));
                        } catch (InterruptedException e) {
                            throw new AssertionError(e);
                        }
                    }
                }
                @Override
                public void onFailedQueryPhase(SearchContext searchContext) {
                    
                    if (searchContext.timeout().millis() > -1) {
                        if (failedQueryLatch.get().getCount() > 0) {
                            failedQueryLatch.get().countDown();
                        }
                    }
                }
            });
            super.onIndexModule(indexModule);
        }
    }
}