fix(docs): update description for React Email version change

[laurendorman]

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Summary by cubic

Update the “Updating React Email” page description to reflect the correct upgrade path: 5.0 → 6.0 instead of 4.0 → 5.0. Keeps the page accurate for readers and search snippets.

^{Written for commit 5ea6aef. Summary will update on new commits.}

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[Copilot]

Pull request overview

Updates the docs frontmatter metadata for the “Updating React Email” guide so the page description matches the current upgrade path covered in the content.

Changes:

• Updated the page description from “React Email 4.0 to 5.0” to “React Email 5.0 to 6.0”.

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[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.