Fix user_sorts_issues_spec when vue_issuables_list ff is off

What does this MR do?

Removes the FF for :vue_issuables_list in spec/features/issues/user_sorts_issues_spec.rb and adds :jss to the rspec because the issues list is now rendered using Vue.

Fixes part of #224038 (closed)

Related backend issue #233420 (closed)