Fix: NotificationPolicy LoopDetected condition and route.receiver validation

[Baarsgaard]

While working on #1837 on I was unable to provoke the LoopDetected status and discovered that it was removed immediately after being set, the detection itself seems to work without issues.
The above resulted in loops being detected, dynamic routes being ignored and an ApplySucceeded condition on GrafanaNotificationPolicy CRs
This means that previously working NotificationPolicy trees with dynamic routes would be overwritten with only the static routes in the main GrafanaNotificationPolicy, if any.

This changes the behaviour from applying only static routes and ignoring everything dynamic to refusing updates entirely.
LoopDetected is now before fetching instances resulting in it having higher priority.
InvalidSpec is an error and the NoMatchingInstances is not.

receiver is now a required field due to the current implementation always returning 400 BAD REQUEST when trying to apply a route when the receiver is omitted.
Lastly, the grafana-operator category was missing from the GrafanaNotificationPolicyRoute

[theSuess]

Good catch, I thought this was covered in the E2E tests

left a small comment but good to merge afterward

[Baarsgaard]

I noticed that status.discoveredRoutes is unstable and the list is slightly random when multiple routes match a single selector.
Added sort to prevent unnecessary status changes.

[msvechla]

Thanks for catching this from my previous PR