Fix `circleci_schedule` resource attributes

[kelvintaywl]

Currently, the various attributes for circleci_schedule are defined as Lists:

• https://registry.terraform.io/providers/kelvintaywl/circleci/latest/docs/resources/schedule#hours_of_day
• https://registry.terraform.io/providers/kelvintaywl/circleci/latest/docs/resources/schedule#days_of_month
• https://registry.terraform.io/providers/kelvintaywl/circleci/latest/docs/resources/schedule#days_of_week
• https://registry.terraform.io/providers/kelvintaywl/circleci/latest/docs/resources/schedule#months

These should be Set, not List. Fixing this should 1. ensure uniqueness of items out-of-the-box, and 2. ensure ordering does not matter.

(2) would likely fix the detected changes similar to what was found on #45

[kelvintaywl]

tested and noted these lists do not suffer the same issue as webhook per #45.

order is maintained somehow even though we did not sort the list(s) from the API response itself. e.g., https://github.com/kelvintaywl/terraform-provider-circleci/blob/e7d728f56d5af1abbe5fa21e9c09c22fcec1d385/internal/provider/schedule_resource.go#L295-L306

[kelvintaywl]

Marking this as wontfix for now, as it's not urgent.