Set cross-version-config-tests to allow_failure in CI

Set cross-version-config-tests to allow_failure in CI

The December releases suffer from the ns2/managed1.conf file not being
in the mkeys extra_artifacts. This manifests only when pytest is run
with the --setup-only option, which is the case in the
cross-version-config-tests CI job. The original issue is fixed in !9815,
but the fix will be effective only when subsequent releases are out.

The #4666 issue removed the "fixed" value for the "rrset-order" option
which is still present in the December release system test and which the
current named can't handle. This will be addressed when when the January
9.21 release is published.

The #4482 issue removed the "dnssec-must-be-secure" feature.