CLN: Deprecate dayofweek/hello day_of_week (#9606)
Rename dayofweek properties in Period, Datetime, and PeriodArray classes.
Add day_of_week property to respective tests for each class.

- [x] closes #9606
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
