ENH: allow Panel.shift on items axis
I'm not sure why `Panel.shift` currently raises with `axis='items'`, while `axis=0` works.

This PR enables `axis='items'` and also updates the docstring to show the parameter `periods` instead of the deprecated `lags`

looks fine. prob some leftover code :)

pls add a release note
if you don't see test for all axes, can you add them (to cover the bases).
 - added a release note
- there were tests for `major` and `minor`, and I've added a test for `items`  
 aweseom. ping on green.
 @jreback build is green now
 Thanks @mortada !
