Remove remaining ImmutableOpen maps from shard stores response (#86427)

The IndicesShardStoresResponse had ImmutableOpenInt map removed, but its
use accidentally remained in serialization. Thankfully the format for
immutable open maps are interchangable with read java Map. This commit
switches to using the new readImmutableMap to read the store statuses.

relates #86239

server/src/main/java/org/elasticsearch/action/admin/indices/shards/IndicesShardStoresResponse.java

@@ -12,8 +12,6 @@
 import org.elasticsearch.action.ActionResponse;
 import org.elasticsearch.action.support.DefaultShardOperationFailedException;
 import org.elasticsearch.cluster.node.DiscoveryNode;
-import org.elasticsearch.common.collect.ImmutableOpenIntMap;
-import org.elasticsearch.common.collect.ImmutableOpenMap;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.io.stream.Writeable;
@@ -241,19 +239,15 @@ public IndicesShardStoresResponse(Map<String, Map<Integer, List<StoreStatus>>> s
     }
 
     IndicesShardStoresResponse() {
-        this(ImmutableOpenMap.of(), Collections.emptyList());
+        this(Map.of(), Collections.emptyList());
     }
 
     public IndicesShardStoresResponse(StreamInput in) throws IOException {
         super(in);
-        storeStatuses = in.readImmutableOpenMap(StreamInput::readString, i -> {
-            int indexEntries = i.readVInt();
-            ImmutableOpenIntMap.Builder<List<StoreStatus>> shardEntries = ImmutableOpenIntMap.builder();
-            for (int shardCount = 0; shardCount < indexEntries; shardCount++) {
-                shardEntries.put(i.readInt(), i.readList(StoreStatus::new));
-            }
-            return shardEntries.build();
-        });
+        storeStatuses = in.readImmutableMap(
+            StreamInput::readString,
+            i -> i.readImmutableMap(StreamInput::readInt, j -> j.readImmutableList(StoreStatus::new))
+        );
         failures = Collections.unmodifiableList(in.readList(Failure::readFailure));
     }