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/plugintest/doc.go

@@ -7,5 +7,5 @@
 // https://godoc.org/github.com/stretchr/testify/mock
 //
 // If you need to import the mock package, you can import it with
-// "github.com/mattermost/mattermost-server/v5/plugin/plugintest/mock".
+// "github.com/mattermost/mattermost-server/v6/plugin/plugintest/mock".
 package plugintest