Deprecate automatic migration location detection for plugins

https://github.com/CTFd/CTFd/blob/ae29dca881a0b1365be1e7ba639d8b36cac30ebc/CTFd/plugins/migrations.py#L56-L62

This strategy of automatically detecting the caller plugin in order to be able to apply migrations is somewhat fragile. If we could do it some other way it would be great but I think we should just deprecate this functionality and let the plugin request its own migrations.

Note that this strategy is used in multiple plugin migration functions.