Rename group import/export config to legacy

With the ndjson introduction, the current group import/export
configuration will be deprecated. This is required because the current
configuration doesn't export the groups `parent_id`, which is required
to map the group tree when using ndjson.

lib/gitlab/import_export.rb

@@ -88,8 +88,8 @@ module Gitlab
       'group.json'
     end
 
-    def group_config_file
-      Rails.root.join('lib/gitlab/import_export/group/import_export.yml')
+    def legacy_group_config_file
+      Rails.root.join('lib/gitlab/import_export/group/legacy_import_export.yml')
     end
   end
 end

lib/gitlab/import_export/group/legacy_tree_restorer.rb

@@ -122,7 +122,7 @@ module Gitlab
           @reader ||= Gitlab::ImportExport::Reader.new(
             shared: @shared,
             config: Gitlab::ImportExport::Config.new(
-              config: Gitlab::ImportExport.group_config_file
+              config: Gitlab::ImportExport.legacy_group_config_file
             ).to_h
           )
         end

lib/gitlab/import_export/group/legacy_tree_saver.rb

@@ -43,7 +43,7 @@ module Gitlab
           @reader ||= Gitlab::ImportExport::Reader.new(
             shared: @shared,
             config: Gitlab::ImportExport::Config.new(
-              config: Gitlab::ImportExport.group_config_file
+              config: Gitlab::ImportExport.legacy_group_config_file
             ).to_h
           )
         end

spec/lib/gitlab/import_export/relation_tree_restorer_spec.rb

@@ -156,7 +156,7 @@ describe Gitlab::ImportExport::RelationTreeRestorer do
     let(:reader) do
       Gitlab::ImportExport::Reader.new(
         shared: shared,
-        config: Gitlab::ImportExport::Config.new(config: Gitlab::ImportExport.group_config_file).to_h
+        config: Gitlab::ImportExport::Config.new(config: Gitlab::ImportExport.legacy_group_config_file).to_h
       )
     end