System Notification Events showing as disabled after adding a notification

## Expected Behavior

The event definition for System Notification Events should always be enabled.
Adding/removing event notifications should not make any difference.

## Current Behavior

System Notification Events is showing as disabled after adding a notification.

`PUT events/definitions/{id}` is being called by the frontend with the (wrong) default state of `DISABLED`.
The backend apparently does not perform any special case checking for System Notification Events, and it ends up being disabled.

The events are still being fired - but the UI showing it disabled is very confusing.
Workaround: It can be reset to enabled using bulk actions (which shouldn't really be supported for this event definition either).

We should probably backport this to 5.x

## Possible Solution
We already modify the event definition form in the frontend for System Notification Events. Set the status to `ENABLED` in the PUT API call.

## Your Environment

* Graylog Version: 6.0.3
* Java Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version: