### 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/internalClusterTest/java/org/elasticsearch/action/search/CCSPointInTimeIT.java
indexDocs(remoteClient, "remote_test", remoteNumDocs);
        boolean includeLocalIndex = randomBoolean();
        List<String> indices = new ArrayList<>();
        if (includeLocalIndex) {
            indices.add(randomFrom("*", "local_*", "local_test"));
        }
        indices.add(randomFrom("*:*", "remote_cluster:*", "remote_cluster:remote_test"));
        String pitId = openPointInTime(indices.toArray(new String[0]), TimeValue.timeValueMinutes(2));
        try {
            if (randomBoolean()) {
                localClient.prepareIndex("local_test").setId("local_new").setSource().get();
                localClient.admin().indices().prepareRefresh().get();
            }
            if (randomBoolean()) {
                remoteClient.prepareIndex("remote_test").setId("remote_new").setSource().get();
                remoteClient.admin().indices().prepareRefresh().get();
            }
            SearchResponse resp = localClient.prepareSearch()
                .setPreference(null)
                .setQuery(new MatchAllQueryBuilder())
                .setPointInTime(new PointInTimeBuilder(pitId))
                .setSize(1000)
                .get();
            assertNoFailures(resp);
            assertHitCount(resp, (includeLocalIndex ? localNumDocs : 0) + remoteNumDocs);
            SearchResponse.Clusters clusters = resp.getClusters();
            int expectedNumClusters = 1 + (includeLocalIndex ? 1 : 0);
            assertThat(clusters.getTotal(), equalTo(expectedNumClusters));
            assertThat(clusters.getClusterStateCount(SearchResponse.Cluster.Status.SUCCESSFUL), equalTo(expectedNumClusters));
            assertThat(clusters.getClusterStateCount(SearchResponse.Cluster.Status.SKIPPED), equalTo(0));