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
```

services/cache/lru.go

@@ -24,7 +24,7 @@ type LRU struct {
 	items                  map[string]*list.Element
 	defaultExpiry          time.Duration
 	name                   string
-	invalidateClusterEvent string
+	invalidateClusterEvent model.ClusterEvent
 }
 
 // LRUOptions contains options for initializing LRU cache
@@ -32,7 +32,7 @@ type LRUOptions struct {
 	Name                   string
 	Size                   int
 	DefaultExpiry          time.Duration
-	InvalidateClusterEvent string
+	InvalidateClusterEvent model.ClusterEvent
 	// StripedBuckets is used only by LRUStriped and shouldn't be greater than the number
 	// of CPUs available on the machine running this cache.
 	StripedBuckets int
@@ -128,7 +128,7 @@ func (l *LRU) Len() (int, error) {
 }
 
 // GetInvalidateClusterEvent returns the cluster event configured when this cache was created.
-func (l *LRU) GetInvalidateClusterEvent() string {
+func (l *LRU) GetInvalidateClusterEvent() model.ClusterEvent {
 	return l.invalidateClusterEvent
 }