Since ActiveSupport::BufferedLogger was deprecated, it's not possible to subclass it anymore. Creating a subclass fails with the following exception:
TypeError: superclass must be a Class (ActiveSupport::Deprecation::DeprecatedConstantProxy given)
This PR applies the same solution we used for ActiveSupport::BasicObject in #8518
There is a difference to BasicObject, which was only subclassed. There could be applications using BufferedLogger instances and I think this should also result in deprecation warnings. I'm not sure how BufferedLogger was used but I'll happily update the PR when we know what else we need to deprecate.
This is a fix for #8576
ActiveSupport::BufferedLogger can be subclassed