DEV: Deprecate message bus site settings (PR #14465)

It makes much more sense for these to be GlobalSettings, since, in multisite clusters, only the default site’s settings would be respected.


What action should be taken when people see this warning? Reset the settings to their default value in the UI?

Whatever the action is, maybe we should include brief instructions in the message?

As annoying as it is, we unfortunately have to deprecate across releases to support people on the stable branch. Discourse.deprecate provides the drop_from and since keyword arguments to make this easier.