rem: ci: Drop FreeBSD 13 support

FreeBSD 13 has reached its end of life, so stop testing on it: remove
the CI build, system test, and unit test jobs together with the runner
tag anchor they used, and move the release from the regularly tested
platforms to the best-effort list in the ARM.

The is_host_freebsd_13 helper existed solely to grant one rerun to two
tests that were flaky on that release, so it goes away with them.

Assisted-by: Claude:claude-opus-5[1m]
Merge branch 'mnowak/drop-freebsd-13' into 'main'

See merge request isc-projects/bind9!12464