Add cnameStrategy: Follow to ACME DNS-01 solvers (#331)

A live DNS-01 smoke test returned NOTAUTH because the solver walked _acme-challenge.unkin.net to zone unkin.net and sent the rfc2136 UPDATE there, but bind-external is only authoritative for acme.unkin.net; without cnameStrategy: Follow the solver does not chase the delegation CNAME.

- Set `cnameStrategy: Follow` on the `letsencrypt` and `letsencrypt-staging` ClusterIssuer DNS-01 solvers so cert-manager follows the `_acme-challenge.unkin.net -> _acme-challenge.acme.unkin.net` CNAME and updates the `acme.unkin.net` zone.

https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT
---------

Co-authored-by: Ben Vincent <neotheo@gmail.com>
Reviewed-on: #331
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

apps/base/cert-manager/clusterissuer_letsencrypt-staging.yaml

@@ -11,6 +11,7 @@ spec:
       name: letsencrypt-staging-account-key
     solvers:
       - dns01:
+          cnameStrategy: Follow
           rfc2136:
             nameserver: "198.18.199.53:53"
             tsigKeyName: certmanager

apps/base/cert-manager/clusterissuer_letsencrypt.yaml

@@ -11,6 +11,7 @@ spec:
       name: letsencrypt-account-key
     solvers:
       - dns01:
+          cnameStrategy: Follow
           rfc2136:
             nameserver: "198.18.199.53:53"
             tsigKeyName: certmanager