### 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
assertTrue(searchResponseAfterCancellation.isRunning());
            assertFalse(searchResponseAfterCancellation.getSearchResponse().isTimedOut());
            assertThat(searchResponseAfterCancellation.getSearchResponse().getClusters().getTotal(), equalTo(2));
            assertThat(searchResponseAfterCancellation.getSearchResponse().getFailedShards(), equalTo(0));
            AsyncStatusResponse statusResponse = getAsyncStatus(response.getId());
            assertTrue(statusResponse.isPartial());
            assertTrue(statusResponse.isRunning());
            assertThat(statusResponse.getClusters().getTotal(), equalTo(2));
            assertThat(statusResponse.getFailedShards(), equalTo(0));
            assertNull(statusResponse.getCompletionStatus());
        } finally {
            SearchListenerPlugin.allowQueryPhase();
        }
        assertBusy(() -> assertTrue(cancelFuture.isDone()));
        waitForSearchTasksToFinish();
        AsyncStatusResponse statusResponseAfterCompletion = getAsyncStatus(response.getId());
        assertTrue(statusResponseAfterCompletion.isPartial());
        assertFalse(statusResponseAfterCompletion.isRunning());
        assertThat(statusResponseAfterCompletion.getClusters().getTotal(), equalTo(2));
        assertThat(statusResponseAfterCompletion.getFailedShards(), greaterThan(0));
        assertThat(statusResponseAfterCompletion.getCompletionStatus(), equalTo(RestStatus.BAD_REQUEST));
        AsyncSearchResponse searchResponseAfterCompletion = getAsyncSearch(response.getId());
        assertTrue(searchResponseAfterCompletion.isPartial());
        assertFalse(searchResponseAfterCompletion.isRunning());
        assertFalse(searchResponseAfterCompletion.getSearchResponse().isTimedOut());
        assertThat(searchResponseAfterCompletion.getSearchResponse().getClusters().getTotal(), equalTo(2));
        assertThat(searchResponseAfterCompletion.getSearchResponse().getFailedShards(), greaterThan(0));
        Throwable cause = ExceptionsHelper.unwrap(searchResponseAfterCompletion.getFailure(), TaskCancelledException.class);
        assertNotNull("TaskCancelledException should be in the causal chain", cause);
        ShardSearchFailure[] shardFailures = searchResponseAfterCompletion.getSearchResponse().getShardFailures();