Remove usage of deprecated `except!` method
https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/120755304
DEPRECATION WARNING: Method except! is deprecated and will be removed in Rails 5.1, as ActionController::Parameters
no longer inherits from hash. Using this deprecated behavior exposes potential security problems. If you continue to use this method you may be creating a security vulnerability in your app that can be exploited. Instead, consider using one of these documented methods which are not deprecated: http://api.rubyonrails.org/v5.0.7/classes/ActionController/Parameters.html (called from execute at /builds/gitlab-org/gitlab-ee/app/services/merge_requests/update_service.rb:10)