pmb.helpers.git.get_branches_official: accept Alpine stable branches
Currently when you run 'pmbootstrap aportgen --fork $package' on v20.05
branch, the Alpine 3.12-stable branch gets checked out. But running
'pmbootstrap pull' afterwards means that the branch doesn't get updated
because that branch isn't accepted by the pull code.
Add 3.12-stable and a couple of releases afterwards to the accepted
branches for aports_upstream.