MM-25516: Changed to byte slice instead of string for cluster messages (#17998)

* MM-25516: Changed to byte slice instead of string for cluster messages

https://mattermost.atlassian.net/browse/MM-25116

Testing:
Manually tested.
Load-tested with Cluster Controller.

I looked into changing the serialization method to use msgpack,
but the ClusterMessage struct was mainly used for only 3 fields
which didn't lead to much of a CPU time improvement, whereas
actually led to more allocations using msgpack. Hence, I chose
to remain with JSON.

```
name              old time/op    new time/op    delta
ClusterMarshal-8    3.51µs ± 1%    3.10µs ± 2%  -11.59%  (p=0.000 n=9+10)

name              old alloc/op   new alloc/op   delta
ClusterMarshal-8      776B ± 0%     1000B ± 0%  +28.87%  (p=0.000 n=10+10)

name              old allocs/op  new allocs/op  delta
ClusterMarshal-8      12.0 ± 0%      13.0 ± 0%   +8.33%  (p=0.000 n=10+10)
```

```release-note
Changed the field type of Data in model.ClusterMessage to []byte from string.
```

* Trigger CI
```release-note
NONE
```

services/users/session.go

@@ -99,7 +99,7 @@ func (us *UserService) ClearUserSessionCache(userID string) {
 		msg := &model.ClusterMessage{
 			Event:    model.ClusterEventClearSessionCacheForUser,
 			SendType: model.ClusterSendReliable,
-			Data:     userID,
+			Data:     []byte(userID),
 		}
 		us.cluster.SendClusterMessage(msg)
 	}

services/users/users.go

@@ -216,7 +216,7 @@ func (us *UserService) InvalidateCacheForUser(userID string) {
 		msg := &model.ClusterMessage{
 			Event:    model.ClusterEventInvalidateCacheForUser,
 			SendType: model.ClusterSendBestEffort,
-			Data:     userID,
+			Data:     []byte(userID),
 		}
 		us.cluster.SendClusterMessage(msg)
 	}