Use Estonian reference number format instead of ISO 11649

lib/tasks/data_migrations/regenerate_registrar_reference_numbers.rake

@@ -0,0 +1,18 @@
+namespace :data_migrations do
+  task regenerate_registrar_reference_numbers: [:environment] do
+    processed_registrar_count = 0
+
+    Registrar.transaction do
+      Registrar.all.each do |registrar|
+        next unless registrar.reference_no.start_with?('RF')
+
+        registrar.reference_no = Billing::ReferenceNo.generate
+        registrar.save!
+
+        processed_registrar_count += 1
+      end
+    end
+
+    puts "Registrars processed: #{processed_registrar_count}"
+  end
+end