Bump custom_cni test Cilium version to 1.18.6

Upgrade the Cilium version used in custom_cni tests from 1.16.3 to
1.18.6 to align with the current cilium_version in Kubespray.

- Update custom_cni_chart_version in debian12-custom-cni-helm.yml
- Regenerate static manifest in tests/files/custom_cni/cilium.yaml
  using helm template cilium/cilium --version 1.18.6

Signed-off-by: thc1006 <84045975+thc1006@users.noreply.github.com>

Author: thc1006