### 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
false,
                    null,
                    1
                ),
                null,
                20,
                9,
                2,
                0,
                ShardSearchFailure.EMPTY_ARRAY,
                createCCSClusterObject(
                    4,
                    3,
                    true,
                    1,
                    1,
                    1,
                    1,
                    new ShardSearchFailure[] { new ShardSearchFailure(new IllegalStateException("corrupt index")) }
                )
            );
            String expectedString = XContentHelper.stripWhitespace("""
                {
                  "took": 0,
                  "timed_out": false,
                  "_shards": {
                    "total": 20,
                    "successful": 9,
                    "skipped": 2,
                    "failed": 0