pmaports.cfg: require pmbootstrap 1.28.0 (MR 1522) (MR 1521)

This version doesn't write /etc/os-release anymore in 'pmbootstrap
install'.

(cherry picked from commit 0a9ac245)

pmaports.cfg

 # Reference: https://postmarketos.org/pmaports.cfg
 [pmaports]
 version=7
-pmbootstrap_min_version=1.22.0
+pmbootstrap_min_version=1.28.0
 channel=stable