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
```

plugin/environment_test.go

@@ -11,7 +11,7 @@ import (
 
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 func TestAvaliablePlugins(t *testing.T) {