Drop FreeBSD 12

Support for FreeBSD 12.4, the last FreeBSD 12.x release, ended on December 31, 2023.

Link: https://www.freebsd.org/security/unsupported/

Move the --with-readline=editline ./configure option to FreeBSD 14.

Also see https://gitlab.isc.org/isc-projects/images/-/merge_requests/296.

Keep FreeBSD 12-based "stress" tests, as we are not ready to upgrade.