📝 Refine upgrade guide

[denialhaag]

Description

🤖 AI text below 🤖

Align the shared upgrade notes with MQT QCEC's final wording and section order.

AI notice

This PR and its contents were created with the assistance of GPT-6 Astra via Codex.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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