Admin::SessionsController#create: cannot execute UPDATE in a read-only transaction

Summary

It happens right after entering an invalid 2FA code to enable Admin mode on a Geo secondary node. It seems a regression from #211456 (closed).

ActiveRecord::StatementInvalid - PG::ReadOnlySqlTransaction: ERROR:  cannot execute INSERT in a read-only transaction:
  ee/app/services/ee/audit_event_service.rb:149:in `unauth_security_event'
  ee/app/controllers/concerns/ee/authenticates_with_two_factor.rb:13:in `log_failed_two_factor'
  app/controllers/concerns/authenticates_with_two_factor_for_admin_mode.rb:101:in `admin_handle_two_factor_failure'
  app/controllers/concerns/authenticates_with_two_factor_for_admin_mode.rb:55:in `admin_mode_authenticate_with_two_factor_via_otp'
  app/controllers/concerns/authenticates_with_two_factor_for_admin_mode.rb:31:in `admin_mode_authenticate_with_two_factor'
  app/controllers/admin/sessions_controller.rb:23:in `create'

Steps to reproduce

1. Enable Admin Mode
2. Enable Two-Factor Authentication for your admin account
3. Enter Admin Mode on a Geo secondary node
4. Enter the correct admin account password
5. Enter an invalid 2FA code

Affected versions

Cross-posting !65504 (comment 620747584):

The audit event for failed 2FA login attempt was introduced in %13.5 see !41641 (merged), but admin mode was only released on %13.10 see https://about.gitlab.com/releases/2021/03/22/gitlab-13-10-released. I'll add the proper labels if we have patch releases for %13.11, %13.12, and %14.0.