[9.18] chg: ci: Enable automatic backports for security fixes

Ensure the "backports" CI job is created when new changes are merged
into security-* branches.  This enables using backport automation for
security fixes.

Backport of MR !11938

Merge branch 'backport-michal/extend-automatic-backports-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11986