docs: update migration guide with send.mime change

[maynkxx]

Summary

This PR updates the Express 4 migration guide to include information about the send.mime change introduced in [email protected].

Details

The send module removed its default MIME type fallback in version 0.18.0. Applications that relied on implicit MIME type detection must now configure MIME behavior manually.

Changes Made

• Updated en/guide/migrating-4.md to document the send.mime fallback removal.
• Added a reference link to the official send release notes.

Related Issue

Fixes #1868

Checklist

• Documentation updated
• Verified formatting and Markdown consistency
• References provided for upstream module change

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🚦 Lighthouse Results (Mobile & Desktop)

URL	Device	Perf	A11y	Best Practices
/	mobile	🟠 87	🟢 100	🟢 96
/en/blog/posts.html	mobile	🟢 97	🟢 96	🟢 96
/en/5x/api.html	mobile	🟢 97	🟢 95	🟢 96
/	desktop	🟢 100	🟢 100	🟢 96
/en/blog/posts.html	desktop	🟢 100	🟢 96	🟢 92
/en/5x/api.html	desktop	🟢 100	🟢 95	🟢 96

[maynkxx]

Hi , this PR adds documentation for the send.mime removal in the migration guide as discussed in issue #1868.
All checks have passed successfully.

Please review when you have a moment. Thank you!

[efekrskl]

Thanks for the PR! It looks like you removed 501 lines, and added none with your last commit :)