Add registrations.reason_required attribute to /api/v2/instance response (#34280)

app/serializers/rest/instance_serializer.rb

@@ -106,6 +106,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     {
       enabled: registrations_enabled?,
       approval_required: Setting.registrations_mode == 'approved',
+      reason_required: Setting.registrations_mode == 'approved' && Setting.require_invite_text,
       message: registrations_enabled? ? nil : registrations_message,
       min_age: Setting.min_age.presence,
       url: ENV.fetch('SSO_ACCOUNT_SIGN_UP', nil),