fix(NcDateTimePicker): improve design and make it consistent with Nc 30

[st3iny]

☑️ Resolves

• Partially addresses https://github.com/nextcloud/calendar/issues/6324

🖼️ Screenshots

Date picker

(Also take note of the confirm button hover state.)

🏚️ Before	🏡 After

Month picker

🏚️ Before	🏡 After

Year picker

🏚️ Before	🏡 After

Time picker

🏚️ Before	🏡 After

🏁 Checklist

• [ ] ⛑️ Tests are included or are not applicable
• [ ] 📘 Component documentation has been extended, updated or is not applicable
• [x] 3️⃣ Backport to next requested with a Vue 3 upgrade

[st3iny]

/backport to next

[susnux]

Seems to be stale and for next we already have a new design