MM-53228: Adding schema version to cluster info (#24209)

With the schema version available, a job can query for
the cluster info to confirm whether or not all nodes
in a cluster are upgraded to the same version or not.

This will help it in determining whether to start
the job or not.

https://mattermost.atlassian.net/browse/MM-53228

```release-note
NONE
```

server/public/model/cluster_info.go

@@ -4,9 +4,10 @@
 package model
 
 type ClusterInfo struct {
-	Id         string `json:"id"`
-	Version    string `json:"version"`
-	ConfigHash string `json:"config_hash"`
-	IPAddress  string `json:"ipaddress"`
-	Hostname   string `json:"hostname"`
+	Id            string `json:"id"`
+	Version       string `json:"version"`
+	SchemaVersion string `json:"schema_version"`
+	ConfigHash    string `json:"config_hash"`
+	IPAddress     string `json:"ipaddress"`
+	Hostname      string `json:"hostname"`
 }