Deprecate file_system_settings_submit() and move to \Drupal\file\Hook\FileHooks

Seems like a good deprecation

https://www.drupal.org/project/drupal/issues/3514161 📌 Better document how form submit callbacks should be called Active

Here are some details, I'll try to connect with berdir and build a list of best practices.

Interesting. I'm not sure how autowired classes will work when using a classname. I was sure they were for static methods, but lets see what the tests say.

I'm beginning to wonder if these belong in a helper class as a static callback.

I changed this to a static method, but seems strange to me that a submit handler must be static.

To be clear the concern I think berdir raises is memory usage, not whether it works.

The way you had it will work, but the whole object is stored.

One final suggestion.

I think you're good to self rtbc once that is in I was about to when I noticed it was method not function.

I think the deprecation is a courtesy, it's a submit callback.

Versions are right.

Applied suggestion and self-RTBC as per #10