Deprecate `offset` property of `Timestamp`

Weird to deprecate offset keyword argument and not offset property. See #6813 for more context.  If we do this, need to:
1. Add getter/setter/deleter property that warns on usage.
2. Migrate all code that refers to `Timestamp.offset` to use `Timestamp.freq` instead.

Then we could remove a few versions from now (hopefully pre-1.0)
