Enable `mail_room`'s structured logging in GitLab

Once structured logging support has been merged into mail_room for #30070 (closed), the next step is to set up GitLab to use it.

As per @engwan's comment:

1. Update omnibus to handle configuration to enable logging and log rotation for this new log file
2. Update gitlab_fluentd to handle this log file
3. Update GitLab CE/EE docs
4. Update GitLab.com runbooks

/cc @gweaver