pkgbase: use <major>.<minor> format for alpha, beta and release package versions

This is consistent with version numbers used in releng/13.2.

PR: 275051
Reviewed by: bapt
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42562

(cherry picked from commit 21e9018ae19662db643a21064150da866bc7beb4)