Archive labels: remove :labels_archive feature flag in frontend

What does this MR do and why?

This MR removes the feature flag :labels_archive from the frontend, thus enabling the archival of labels. We have previously removed it for the backend.

🛠️ with ❤️ at Siemens

References

• Issue: #4233
• Feature Flag: #556700
• Follow up to: !211559 (merged)

Screenshots or screen recordings

How to set up and validate locally

1. With the GDK seed data, you can go to https://gdk.test:3443/gitlab-org/gitlab-shell/-/labels. Choose Archive in the Three-dot-menu to archive a label.
2. It should now show up in the Archive tab.
3. Now, in the same project, go to an existing issue or create a new issue. In the labels dropdown, you should no longer see the label you've archived. You should also not be able to add it with /label ~your-archived-label
4. You can Unarchive labels either by going to the edit view or by using the three-dot-menu from the Archived tab.

This should work for Group and Project labels. The entry must not be visible for Admin labels, as they are template labels and cannot be archived.

MR acceptance checklist

MR Checklist ( @nwittstruck)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

Related to #4233