[9.18] fix: ci: remove allow failure in cross version config tests

From https://gitlab.isc.org/isc-projects/bind9/-/issues/5087, the relevant MRs have been merged in the January 2025 release. Hence this MR removes `allow_failure: true` in CI.

Backport of MR !10026

Merge branch 'backport-andoni/remove-allow-failure-in-cross-version-config-tests-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10028