Get rid of cattr_accessor in ActiveRecord::Base

[casperisfine]

See #42442 for full context.

In short class variables are slow, particularly in classes with many ancestors, and ActiveRecord::Base has over 60 ancestors.

Only a small handful of these really matter performance wise, however the idea is to migrate them all regardless for consistency and to ensure future contributors will follow the new pattern rather than add new mattr/cattr.

This should be the last ones in ActiveRecord::Base. There are others in other Active Record classes, but since they don't have large ancestor chains, it doesn't matter much.

I'll look at other classes with large ancestors chains instead (ActionController::Base ?)