Deprecate offset property of Timestamp

[jtratner]

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)

[jreback]

moving to #6581 (the master issue for deprecations)