chore(docs): mention removed post-button variables in the migration guide

[hugomslv]

📄 Description

Append removal of old button variables to the migration guide

🚀 Demo

If applicable, please add a screenshot or video to illustrate the changes.

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📝 Checklist

• ✅ My code follows the style guidelines of this project
• 🛠️ I have performed a self-review of my own code
• 📄 I have made corresponding changes to the documentation
• ⚠️ My changes generate no new warnings or errors
• 🧪 I have added tests that prove my fix is effective or that my feature works
• ✔️ New and existing unit tests pass locally with my changes