Resolve "Delete `Projects::IssuesController#redirect_old`"

What does this MR do?

It simply removes the issues_controller.rb#redirect_old

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • All builds are passing
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #20517 (closed)