ci: update to freebsd 13.1 / remove minor versions from image names

Hi,

Freebsd 13.0, so far used by CI, is out of support. I've changed the
image to be built against 13.1, so we can switch to that.

I suspect it'd be better to remove the minor version numbers from the
image name, so that switches from 13.0 -> 13.1 don't require CI
changes. Any argument against?

I can also see an argument for not having 13 in the image name, given
that the image is CI specific anyway? But perhaps we might want to have
a 13 and a 14 image for some debugging issue?

Greetings,

Andres Freund