DEPR: rename Timestamp.offset to .freq
- [x] closes #12160
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13593?src=pr) is **84.31%**

> No coverage report found for **master** at 1edc1df.
> 
> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [1edc1df...c7749d5](https://codecov.io/gh/pydata/pandas/compare/1edc1df161f3274218fcd19c23663ea63386f105...c7749d52d61a4de90e3910b8ae431d62092f3847?src=pr)
 thanks @sinhrks 

I had to do a couple of more renames to avoid the warnings...but no biggie.

not that `Timestamp.from_ordinal` also needed changing, but I don't think its tested much.
 Thanks, let me do a follow-up for testing `fromordinal`.
