chg: ci: Split ci/freebsd.ini into per-release machine files

FreeBSD 15 replaced base Heimdal with MIT Kerberos 5, so pointing
krb5-config at the MIT port is no longer necessary there; FreeBSD 14
still ships Heimdal and needs the override.  A single shared machine
file cannot express that difference, so give each release its own.

Assisted-by: Claude:claude-opus-5[1m]
Merge branch 'mnowak/split-freebsd-ci-machine-files' into 'main'

See merge request isc-projects/bind9!12510