c++20 std::multimap now has a contains method which is preferred over…

… count

cpp/mrc/src/internal/control_plane/server/connection_manager.cpp

@@ -113,7 +113,7 @@ Expected<protos::RegisterWorkersResponse> ConnectionManager::register_instances(
     }
 
     // check if any workers/instances have been registered on the requesting stream
-    if (m_instances_by_stream.count(stream_id) != 0)
+    if (m_instances_by_stream.contains(stream_id))
     {
         return Error::create(MRC_CONCAT_STR("failed to register instances on immutable stream "
                                             << stream_id << "; streams are immutable after first registration"));