* 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
```
7be61af24f