pmb.helpers: Handle workdir version suffix from 2.3.x (MR 2447)

For the gitlab.com -> gitlab.postmarketos.org migration we needed to
also migrate the workdir in pmb v2 since pmb v3 is not ready yet.

Therefore we need to handle that "7-2.x" workdir version and migrate
correctly when switching to pmbootstrap v3.

See !2445