pmbootstrap should zap the work dir when migrating from 2.3.x to 3

There is already logic to bump the workdir version and zap it if the version was lower. We should bump the version and require a zap for pmbootstrap v3 too.

(Caleb mentioned that the chroots should be zapped when upgrading to pmb v3 and I've also just seen some errors when not doing that.)