Order VPN articles by first published date descending (Fixes #15517)

[alexgibson]

One-line summary

Renders VPN RC articles on the index page by first_published_at date, descending order. This should ensure that the most newly published articles appear first on the page, with the oldest at the bottom.

Issue / Bugzilla link

15517

Testing

1. Log into the CMS: http://localhost:8000/cms-admin/
2. Create a structural page with the slug of products.
3. Create a child structural page with the slug of vpn.
4. Create a child VPN resource center index page with a slug of resource-center.
5. Create a RC child page e.g. "Test page 1" and publish.
6. Create a RC child page e.g. "Test page 2" and publish.
7. Verify that "Test page 2" is rendered first on the index page, and "Test page 1" is last.
8. Bonus points for verifying that draft pages always go to the top of the list when published, irrespective of when they were drafted.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.78%. Comparing base (b7b5dee) to head (8691e0f). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #15518 +/- ## ======================================= Coverage 78.78% 78.78% ======================================= Files 157 157 Lines 8257 8257 ======================================= Hits 6505 6505 Misses 1752 1752 ```

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