MM-21357: Use typed constants ClusterEvent (#17920)

* MM-21357: Use typed constants ClusterEvent

Use typed constants for:
- model.ClusterEvent

https://mattermost.atlassian.net/browse/MM-21357

```release-note
Use typed constant for model.ClusterEvent
```

* trigger CI
```release-note
NONE
```

app/plugin_event.go

@@ -7,7 +7,7 @@ import (
 	"github.com/mattermost/mattermost-server/v5/model"
 )
 
-func (s *Server) notifyClusterPluginEvent(event string, data model.PluginEventData) {
+func (s *Server) notifyClusterPluginEvent(event model.ClusterEvent, data model.PluginEventData) {
 	if s.Cluster != nil {
 		s.Cluster.SendClusterMessage(&model.ClusterMessage{
 			Event:            event,