fix: ci: Set cross-version-config-tests to allow_failure in CI

Address failing cross-version-config-tests job.

Closes #5087

Merge branch 'mnowak/cross-version-config-tests-allow-fail' into 'main'

See merge request isc-projects/bind9!9833

.gitlab-ci.yml

@@ -744,6 +744,18 @@ cross-version-config-tests:
     untracked: true
     expire_in: "1 day"
     when: always
+  # Changes in the January milestone necessitate allowing this job to fail. The
+  # "soft failure" should be reverted when January releases are published.
+  # - #4261 introduced extra artifacts check but missed a file visible only in
+  #   this job. This is fixed in !9815 but present in December releases.
+  #   System test affected: mkeys.
+  # - #4666 removed the "fixed" value for the "rrset-order" option, but the
+  #   value is still present in the December release system test.
+  #   System test affected: rrsetorder.
+  # - #4482 removed the "dnssec-must-be-secure" feature that is still present
+  #   in the December release.
+  #   System tests affected: autosign, dnssec, and dsdigest.
+  allow_failure: true
 
 # Jobs for regular GCC builds on Alpine Linux 3.21 (amd64)