fix(sealed-secrets): migrate Helm repo URL to bitnami

bitnami-labs GitHub Pages repo (bitnami-labs.github.io/sealed-secrets)
returns 404 — chart moved to charts.bitnami.com/bitnami. Updates chart
version from 2.16.1 to 2.5.19 (controller 0.27.1 → 0.31.0). Controller
is backward-compatible with existing sealed secrets.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

eu-central-1/argo-apps/sealed-secrets/sealed-secrets.yaml

@@ -9,9 +9,9 @@ spec:
     namespace: kube-system
     server: https://kubernetes.default.svc
   sources:
-    - repoURL: https://bitnami-labs.github.io/sealed-secrets
+    - repoURL: https://charts.bitnami.com/bitnami
       chart: sealed-secrets
-      targetRevision: 2.16.1
+      targetRevision: 2.5.19
       helm:
         valueFiles:
           - $values/sealed-secrets/eu-central-1/values.yaml