Add `Guild.max_stage_video_users` and `Guild.safety_alerts_channel`

[ThatOtherAndrew]

Summary

Add the currently missing Guild.max_stage_video_users attribute and Guild.safety_alerts_channel property

What is the feature request for?

The core library

The Problem

In https://github.com/Rapptz/discord.py/pull/9318, support for Guild.max_stage_video_users and Guild.safety_alerts_channel have been added. It'd be nice to get that in d.py-self as well.

The Ideal Solution

I'm not familiar enough with the codebase (and running a diff of the guild.py files of discord.py and discord.py-self yielded this monstrous diff which was too intimidating for me) so I haven't dared to touch it myself, but in an ideal world the changes made in the PR should be able to be merged into here?

The Current Solution

I don't believe there is any.

Additional Context

No response