main/postmarketos-release-upgrade-notification: new aport

Previously the process was that once a branch is EOL, the notification-commit was cherry-picked from the branch before.

However that makes less sense, since the package then doesn't go through the same mass updates as pmaports master does, e.g. it still referenced gitlab.com & -gtk3, and doesn't allow changes by contributors other than the one cherry-picking it.

Adapted from c088d806 ("main/postmarketos-upgrade-gtk3: new aport")

See #4197.