Fix: Update Set<T> to List<T> in calendar_event.dart to comply with rrule 0.2.16

[sheng-di]

This PR addresses the type declaration change in the rrule library version 0.2.16. The library now uses List<T> instead of Set<T>, which necessitates the same change in calendar_event.dart of the builttoroam/device_calendar package.

Changes made:

• All instances of Set<T> in calendar_event.dart have been updated to List<T>.
• Necessary adjustments in methods/functions that were using Set<T> are made to handle List<T>.

This change ensures compatibility with the rrule library version 0.2.16 and prevents possible type errors. It is recommended to merge this PR to maintain the integrity and functionality of the builttoroam/device_calendar package.

Please review and let me know if any changes are required.

[IVLIVS-III]

@sheng-di thank you for fixing the example project that causes all tests to break. Please rebase your branch on the top of develop. The workflow issue should be fixed by now.