### 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
assertFalse(SearchResponse.Clusters.EMPTY.hasRemoteClusters());
        assertFalse(new SearchResponse.Clusters(1, 1, 0).hasRemoteClusters());
        
        
        assertTrue(new SearchResponse.Clusters(3, 2, 1).hasRemoteClusters());
        {
            Map<String, OriginalIndices> remoteClusterIndices = new HashMap<>();
            remoteClusterIndices.put("remote1", new OriginalIndices(new String[] { "*" }, IndicesOptions.LENIENT_EXPAND_OPEN));
            var c = new SearchResponse.Clusters(null, remoteClusterIndices, randomBoolean(), alias -> randomBoolean());
            assertTrue(c.hasRemoteClusters());
        }
        {
            OriginalIndices localIndices = new OriginalIndices(new String[] { "foo*" }, IndicesOptions.LENIENT_EXPAND_OPEN);
            Map<String, OriginalIndices> remoteClusterIndices = new HashMap<>();
            remoteClusterIndices.put("remote1", new OriginalIndices(new String[] { "*" }, IndicesOptions.LENIENT_EXPAND_OPEN));
            var c = new SearchResponse.Clusters(localIndices, remoteClusterIndices, randomBoolean(), alias -> randomBoolean());
            assertTrue(c.hasRemoteClusters());
        }
        {
            OriginalIndices localIndices = new OriginalIndices(new String[] { "foo*" }, IndicesOptions.LENIENT_EXPAND_OPEN);
            Map<String, OriginalIndices> remoteClusterIndices = new HashMap<>();
            remoteClusterIndices.put("remote1", new OriginalIndices(new String[] { "*" }, IndicesOptions.LENIENT_EXPAND_OPEN));
            remoteClusterIndices.put("remote2", new OriginalIndices(new String[] { "a*" }, IndicesOptions.LENIENT_EXPAND_OPEN));
            remoteClusterIndices.put("remote3", new OriginalIndices(new String[] { "b*" }, IndicesOptions.LENIENT_EXPAND_OPEN));
            var c = new SearchResponse.Clusters(localIndices, remoteClusterIndices, randomBoolean(), alias -> randomBoolean());
            assertTrue(c.hasRemoteClusters());
        }
    }
}