Update route for group work item settings

Issue: Update settings nav items to reference work items (#580271)

What does this MR do and why?

• Adds a new route for group settings -> work items
• Updates link in group settings to use this new route if FF work_item_planning_view is enabled

References

Related to: #580271 (comment 2915754712)

Stacked on this MR: Update group/project settings nav items to refe... (!214206 - merged)

daniyalAD: Similar issue here, the frontend changes will need to be accompanied by redirects on the backend otherwise the url still contains /issues while the page contains work items references.

vshushlin: I don't think anyone bookmarks settings pages, so you can just change the URL. To be absolutely safe, we can alias /issues to /work_items in the routes.

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