### 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
public TestQueryBuilderPlugin() {}
        @Override
        public List<QuerySpec<?>> getQueries() {
            QuerySpec<SlowRunningQueryBuilder> slowRunningSpec = new QuerySpec<>(
                SlowRunningQueryBuilder.NAME,
                SlowRunningQueryBuilder::new,
                p -> {
                    throw new IllegalStateException("not implemented");
                }
            );
            QuerySpec<ThrowingQueryBuilder> throwingSpec = new QuerySpec<>(ThrowingQueryBuilder.NAME, ThrowingQueryBuilder::new, p -> {
                throw new IllegalStateException("not implemented");
            });