AC calc changes

I do not believe the data for AC actually changed, only the way it is calculated.
evidence: no changes were made to the template.json related to AC

Related MR: https://gitlab.com/foundrynet/dnd5e/-/merge_requests/481

thread for this cause i bet it'll be a topic

The migration should change any uses of attributes.ac.base to attributes.ac.armor in custom formulas

and to be clear, the use of base before 1.6.0 just didn't work right? (after some 1.5.x version)
Specifically talking about in active effects I think

I worked somewhat, but there were all sorts of odd results

Oh, and the migration will also alter any active effects that target data.attributes.ac.base and change them