.gitlab-ci.d/cirrus: Drop the CI job for compiling with FreeBSD 12

.gitlab-ci.d/cirrus: Drop the CI job for compiling with FreeBSD 12

FreeBSD 13.0 has been released in April 2021:

 https://www.freebsd.org/releases/13.0R/announce/

According to QEMU's support policy, we stop supporting the previous
major release two years after the the new major release has been
published. So we can stop testing FreeBSD 12 in our CI now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230418160225.529172-1-thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20230424092249.58552-8-alex.bennee@linaro.org>