Update postmaster.mdx

[vitch]

Fix the link to https://developers.cloudflare.com/changelog/2025-06-30-mail-authentication/

Summary

The current link doesn't go anywhere. I manually looked through the changelog and found where it was meant to go to so updated the link...

Screenshots (optional)

Documentation checklist

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PR Change Summary

Updated the link for mail authentication requirements in the documentation to point to the correct changelog entry.

• Fixed the link to the mail authentication changelog entry
• Ensured the documentation accurately reflects the current requirements for email authentication

Modified Files

• src/content/docs/email-routing/postmaster.mdx

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