DEV: Update the rubocop-discourse gem

This enables cops related to RSpec `subject`.

See https://github.com/discourse/rubocop-discourse/pull/32

plugins/poll/spec/lib/polls_validator_spec.rb

@@ -3,8 +3,9 @@
 require "rails_helper"
 
 RSpec.describe ::DiscoursePoll::PollsValidator do
+  subject(:validator) { described_class.new(post) }
+
   let(:post) { Fabricate(:post) }
-  subject { described_class.new(post) }
 
   describe "#validate_polls" do
     it "ensures that polls have valid arguments" do