Remove keepalived_notification_email variable

```text
  TASK [hifis.toolkit.keepalived : Show deprecation warning] *********************
  ok: [instancekeepalived] => 
    msg:
    - The variable keepalived_notification_email is deprecated and will be removed in the next major release.
    - Please use list variable 'keepalived_notification_emails' instead.
```

See https://github.com/hifis-net/ansible-collection-toolkit/pull/391