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/hooks.go

@@ -7,7 +7,7 @@ import (
 	"io"
 	"net/http"
 
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 // These assignments are part of the wire protocol used to trigger hook events in plugins.