Fix deprecation warning in Tagging monkey patch

We were getting a warning in Rails 6.0:

DEPRECATION WARNING: Class level methods will no longer inherit scoping
from `public_for_api` in Rails 6.1. To continue using the scoped
relation, pass it into the block directly. To instead access the full
set of models, as Rails 6.1 will, use
`ActsAsTaggableOn::Tag.default_scoped`.

config/initializers/acts_as_taggable_on.rb

@@ -5,7 +5,8 @@ module ActsAsTaggableOn
 
     scope :public_for_api, -> do
       where(
-        tag: Tag.where(kind: [nil, "category"]),
+        # TODO: remove default_scoped after upgrading to Rails 6.1
+        tag: Tag.default_scoped.where(kind: [nil, "category"]),
         taggable: [Debate.public_for_api, Proposal.public_for_api]
       )
     end