Set minimum required HA version to 2021.12.0

[weltenwort]

📝 Summary

This correctly sets the minimum Home Assistant Core version to 2021.12.0, because the recent migrations from device class and state class constants to enums are only supported since that version.

[github-actions]

🎉 HACS repository validator action summary 🎉
✅ The repository is not archived
✅ manifest.json file exist
✅ All required keys are present in manifest.json
✅ hacs.json has the 'name' key set
✅ The repository has a description
✅ rct_power is added to https://github.com/home-assistant/brands NICE!
✅ The repository has topics
✅ The repository has issues enabled
✅ This day ends with an 'y' (Thursday)
✅ info.md exists
✅ Requirements validation
✅ HACS load check