MM-36271: Bump major version to 6.0 (#17973)

https://mattermost.atlassian.net/browse/MM-36271

```release-note
We bump the major version to 6.0
```

testlib/cluster.go

@@ -6,8 +6,8 @@ package testlib
 import (
 	"sync"
 
-	"github.com/mattermost/mattermost-server/v5/einterfaces"
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/einterfaces"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type FakeClusterInterface struct {