Notifications generate this deprecation error:

```
DEPRECATION WARNING: Sending mail with DeliveryJob and
Parameterized::DeliveryJob is deprecated and will be removed in Rails
6.1. Please use MailDeliveryJob instead. (called from new_project_member
at /builds/gitlab-org/gitlab/app/services/notification_service.rb:360)
```

This was introduced in Rails: https://github.com/rails/rails/pull/34591.

To migrate, we simply need to set the Rails configuration to use
`MailDeliveryJob` instead of `DeliveryJob` and update the code to
reflect that.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/208305