Fix 2.7 warning for sidekiq_middleware_spec.rb

ActiveJob arguments have a special key for kwargs:
https://github.com/rails/rails/blob/8e452c710d9f8bb7dec24407c0c3c5508e26ef01/activejob/lib/active_job/arguments.rb#L55

Let's use this key in this particular test since
it's the intention

spec/lib/gitlab/sidekiq_middleware_spec.rb

@@ -100,7 +100,7 @@ RSpec.describe Gitlab::SidekiqMiddleware do
                   "subject",
                   "body"
                 ],
-                "_aj_symbol_keys" => ["args"]
+                ActiveJob::Arguments.const_get('RUBY2_KEYWORDS_KEY', false) => ["args"]
               }
             ],
             "executions" => 0,