ci: Upgrade macOS version from 13 to 14.

ci: Upgrade macOS version from 13 to 14.

1.  Previously we were using ghcr.io/cirruslabs/macos-XXX-base:latest
images, but Cirrus has started ignoring that and using a particular
image, currently ghcr.io/cirruslabs/macos-runner:sonoma, for github
accounts using free CI resources (as opposed to dedicated runner
machines, as cfbot uses).  Let's just ask for that image anyway, to stay
in sync.

2.  Instead of hard-coding a MacPorts installation URL, deduce it from
the running macOS version and the available releases.  This removes the
need to keep the ci_macports_packages.sh in sync with .cirrus.task.yml,
and to advance the MacPorts version from time to time.

3.  Change the cache key we use to cache the whole macports installation
across builds to include the OS major version, to trigger a fresh
installation when appropriate.

Back-patch to 15 where CI began.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGLqJdv6RcwyZ_0H7khxtLTNJyuK%2BvDFzv3uwYbn8hKH6A%40mail.gmail.com