MINOR: return unmodifiableMap for PartitionStates.partitionStateMap. (#7637)

Makes the map returned by partitionStateMap unmodifiable to prevent mutation.

Reviewers: Ismael Juma <ismael@juma.me.uk>

clients/src/main/java/org/apache/kafka/common/internals/PartitionStates.java

@@ -89,8 +89,8 @@ public class PartitionStates<S> {
         return map.entrySet().stream().map(entry -> new PartitionState<>(entry.getKey(), entry.getValue()));
     }
 
-    public LinkedHashMap<TopicPartition, S> partitionStateMap() {
-        return map;
+    public Map<TopicPartition, S> partitionStateMap() {
+        return Collections.unmodifiableMap(map);
     }
 
     /**