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
```

api4/plugin_test.go

@@ -1758,7 +1758,7 @@ func TestInstallMarketplacePlugin(t *testing.T) {
 	}, "missing prepackaged and remote plugin signatures")
 }
 
-func findClusterMessages(event string, msgs []*model.ClusterMessage) []*model.ClusterMessage {
+func findClusterMessages(event model.ClusterEvent, msgs []*model.ClusterMessage) []*model.ClusterMessage {
 	var result []*model.ClusterMessage
 	for _, msg := range msgs {
 		if msg.Event == event {