Change potentially offensive terminology

I think the few places we use the term "whitelist", we really don't need
to. Changing to use "allowed" etc. instead.

RuboCop did similar changes in
rubocop/rubocop#7469.

spec/rubocop/cop/rspec_rails/http_status_spec.rb

@@ -60,7 +60,7 @@
       RUBY
     end
 
-    it 'does not register an offense when using whitelisted symbols' do
+    it 'does not register an offense when using allowed symbols' do
       expect_no_offenses(<<-RUBY)
         it { is_expected.to have_http_status :error }
         it { is_expected.to have_http_status :success }