MM-60413: Fix issues due to incorrect cache provider (#28181)

We were using a mock cache provider which mixed up LRU caches
with each other. This led to incorrect unmarshalling method calls.

We fix this by using the real cache provider.

https://mattermost.atlassian.net/browse/MM-60413

```release-note
NONE
```

server/platform/services/cache/lru.go

@@ -220,11 +220,6 @@ func (l *LRU) get(key string, value any) error {
 		_, err := u.UnmarshalMsg(val)
 		*v = &u
 		return err
-	case *map[string]*model.User:
-		var u model.UserMap
-		_, err := u.UnmarshalMsg(val)
-		*v = u
-		return err
 	}
 
 	// Slow path for other structs.