chore(vikunja): rename JWT env to non-deprecated service.secret (DEV-560)

Vikunja 2.6.0 deprecates `service.jwtsecret` in favor of `service.secret`;
switch the env var name (secret payload unchanged) and fix the admin-promotion
example in the README to the current CLI (`user set-admin --admin`).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/vikunja/README.md

@@ -25,7 +25,7 @@ Both the app and Postgres pin to Hetzner location `fsn1` because their PVCs live
 
 - Pocket ID client `Vikunja` (id `cdfff4ee-28ba-4aab-9f47-7e41b4b6b847`), callback `https://vikunja.basicstack.de/auth/openid/basicstack`.
 - Pocket ID groups `vikunja_admins` and `vikunja_members`.
-- Vikunja does not map OIDC groups to the admin flag. Promote the first user post-login: `kubectl -n vikunja exec deploy/vikunja -- /app/vikunja/vikunja user chmod --admin <username>`.
+- Vikunja does not map OIDC groups to the admin flag. Promote the first user post-login: `kubectl -n vikunja exec deploy/vikunja -- /app/vikunja/vikunja user set-admin --admin <username>`.
 
 ## Optional: SMTP
 

apps/vikunja/vikunja-deployment.yaml

@@ -51,7 +51,7 @@ spec:
         env:
         - name: VIKUNJA_SERVICE_PUBLICURL
           value: https://vikunja.basicstack.de/
-        - name: VIKUNJA_SERVICE_JWTSECRET
+        - name: VIKUNJA_SERVICE_SECRET
           valueFrom:
             secretKeyRef:
               name: vikunja-app-secrets