Replaced moment with luxon in core/frontend/helpers/prev_post.js

[PakkuDon]

refs: #13648
Moment is in maintenance mode and is no longer recommended for use

• There's a clear use-case for this code change
• Commit message has a short title & references relevant issues
• The build will pass (run yarn test and yarn lint)

[ErisDS]

Hi @PakkuDon thanks for this PR 🙏 I've just been testing it out and although it passes the tests it doesn't actually work. Our tests relied on the fact moment would cope with any type of date and so uses new Date(). Your code uses DateTime.fromJSDate which works with this test data.

However in the real world this results in "Invalid DateTime".

I tried using fromISO, which results in the date being parsed, but the .toFormat() function works differently to moment so this results in YYYY-10-21 Oct 2021 09:02:10 instead of 2021-10-21 09:02:10.

[github-actions]

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