test(rails): gate retry notification specs to Rails 6

Author: solnic

sentry-rails/spec/sentry/rails/active_job_job_extensions_spec.rb

@@ -305,7 +305,7 @@ def register_deciding_extension(report:, will_retry:)
     end
   end
 
-  describe "retried exception reporting" do
+  describe "retried exception reporting", skip: RAILS_VERSION < 6.0 do
     let(:retrying_job) do
       job_fixture do
         retry_on StandardError, attempts: 2, wait: 0