Fix: Update authentication guide to use .ts files instead of .mjs

[Keevh]

Description (required)

Updated the authentication guide to use .ts instead of .mjs for the configuration file. This aligns the documentation with the official library's README and prevents potential errors when users follow the guide.

Related issues & labels (optional)

• Closes auth-astro instructions show using a .mjs #10218
• Suggested label: documentation

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Yes! 🎉
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Tracked Files

File	Note
en/guides/authentication.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

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[apatel369]

Thanks for doing this @Kevin2606. Welcome to Team Docs! 🥳

Your changes are perfect.

[yanthomasdev]

Thanks @Kevin2606, welcome to Team Docs!