⬆️ Raise minimum Qiskit version to 2.1.1

[denialhaag]

Description

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Raise the minimum Qiskit version from 2.0.3 to 2.1.1 and require pytket-qiskit 0.71.1 or newer.

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.
• 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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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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