Use dynamic attributes in factories

Static attributes are deprecated since factory bot 4.11; we haven't
upgraded yet but now we're ready to do so:

https://thoughtbot.com/blog/deprecating-static-attributes-in-factory_bot-4-11

spec/factories/votes.rb

@@ -2,7 +2,7 @@ FactoryBot.define do
   factory :vote do
     association :votable, factory: :debate
     association :voter,   factory: :user
-    vote_flag true
+    vote_flag { true }
     after(:create) do |vote, _|
       vote.votable.update_cached_votes
     end