Fix usage of ActiveRecord::Base in importer code

Use the connection from the associated relation class rather than
ActiveRecord::Base when doing an index check.

https://gitlab.com/gitlab-org/gitlab/-/issues/350191

.rubocop_todo/database/multiple_databases.yml

@@ -20,7 +20,6 @@ Database/MultipleDatabases:
   - lib/gitlab/database.rb
   - lib/gitlab/gitlab_import/importer.rb
   - lib/gitlab/health_checks/db_check.rb
-  - lib/gitlab/import_export/base/relation_factory.rb
   - lib/gitlab/import_export/group/relation_tree_restorer.rb
   - lib/gitlab/legacy_github_import/importer.rb
   - lib/gitlab/seeder.rb

lib/gitlab/import_export/base/relation_factory.rb

@@ -300,7 +300,7 @@ def has_unique_index_on_importable_fk?
           return cache[table_name] if cache.has_key?(table_name)
 
           index_exists =
-            ActiveRecord::Base.connection.index_exists?(
+            relation_class.connection.index_exists?(
               relation_class.table_name,
               importable_foreign_key,
               unique: true)