Fix streaming serializer for serialize_many_each

!28025 (merged) MR got merged together with !27679 (merged). Now we have broken code on master: https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/import_export/json/streaming_serializer.rb#L75 Since method name of json_writer interface changed from append to write_relation_array

This MR fixes that code.

Fixes #212607 (closed).