Attempt to fix DatabaseCleaner for Ruby 3.0 (DatabaseCleaner/database_cleaner-mongoid#16)

Author: ronaldtse

spec/support/db_cleaner.rb

@@ -12,7 +12,9 @@
     # Therefore, they are specified explicitly here.
     unless WITHOUT_MONGOID
       require "database_cleaner-mongoid"
-      DatabaseCleaner[:mongoid].strategy = :deletion, { only: %w[users] }
+      strategy = DatabaseCleaner::Mongoid::Deletion.new(only: %w[users])
+      DatabaseCleaner[:mongoid].instance_variable_set :'@strategy', strategy
+      DatabaseCleaner[:mongoid].start
     end
 
     DatabaseCleaner.clean