### 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/search/ccs/CrossClusterSearchIT.java
private Map<String, Object> setupTwoClusters() {
        String localIndex = "demo";
        int numShardsLocal = randomIntBetween(2, 10);
        Settings localSettings = indexSettings(numShardsLocal, randomIntBetween(0, 1)).build();
        assertAcked(
            client(LOCAL_CLUSTER).admin()
                .indices()
                .prepareCreate(localIndex)
                .setSettings(localSettings)
                .setMapping("@timestamp", "type=date", "f", "type=text")
        );
        indexDocs(client(LOCAL_CLUSTER), localIndex);
        String remoteIndex = "prod";
        int numShardsRemote = randomIntBetween(2, 10);
        final InternalTestCluster remoteCluster = cluster(REMOTE_CLUSTER);
        remoteCluster.ensureAtLeastNumDataNodes(randomIntBetween(1, 3));
        assertAcked(
            client(REMOTE_CLUSTER).admin()
                .indices()
                .prepareCreate(remoteIndex)
                .setSettings(indexSettings(numShardsRemote, randomIntBetween(0, 1)))
                .setMapping("@timestamp", "type=date", "f", "type=text")
        );
        assertFalse(
            client(REMOTE_CLUSTER).admin()
                .cluster()
                .prepareHealth(remoteIndex)
                .setWaitForYellowStatus()
                .setTimeout(TimeValue.timeValueSeconds(10))
                .get()