Members of the `BOT` usergroup shouldn't be able to be assigned guest difficulties

Self-explanatory I guess.

Related to the recent incident on [this map](http://osu.ppy.sh/s/2287978), where the mapset host tried to set the ownership of his map to bot accounts such as "[BanchoBot](http://osu.ppy.sh/u/3)" and "[Nomination Assessment Team](http://osu.ppy.sh/u/6616586)" :

![image](https://github.com/user-attachments/assets/d5e27423-9be6-4c84-aaff-0ca643e91771)
