docs: Mark certManager as non-experimental in guide

Author: Techassi

docs/modules/secret-operator/examples/cert-manager/secretclass.yaml

@@ -5,7 +5,7 @@ metadata:
   name: tls-cert-manager # <1>
 spec:
   backend:
-    experimentalCertManager:
+    certManager:
       issuer:
         kind: Issuer # <2>
         name: secret-operator-demonstration # <3>

docs/modules/secret-operator/examples/secretclass-certmanager-key-length.yaml

@@ -5,7 +5,7 @@ metadata:
   name: tls
 spec:
   backend:
-    experimentalCertManager:
+    certManager:
       issuer:
         kind: Issuer
         name: secret-operator-demonstration

docs/modules/secret-operator/pages/cert-manager.adoc

@@ -3,7 +3,7 @@
 :cert-manager: https://cert-manager.io/
 :letsencrypt: https://letsencrypt.org/
 
-WARNING: The Cert-Manager backend is experimental, and subject to change.
+NOTE: The Cert-Manager backend was marked as non-experimental in SDP 25.11.0
 
 {cert-manager}[Cert-Manager] is a common tool to manage certificates in Kubernetes, especially when backed by an external Certificate Authority (CA) such as {letsencrypt}[Let\'s Encrypt].