ci: Upgrade FreeBSD image

ci: Upgrade FreeBSD image

Upgrade to the current stable version. To avoid needing commits like this in
the future, the CI image name now doesn't contain the OS version number
anymore.

Backpatch to all versions with CI support, we don't want to generate CI images
for multiple FreeBSD versions.

Author: Nazir Bilal Yavuz 
Discussion: https://postgr.es/m/CAN55FZ3_P4JJ6tWZafjf-_XbHgG6DQGXhH-y6Yp78_bwBJjcww@mail.gmail.com
Backpatch-through: 15