Fix: helm template redis (#7563)

### What problem does this PR solve?

Fixes bug & regression introduced by [PR #7187 - refactor: Update Redis
configuration to use StatefulSet instead of deployment with
pvc](https://github.com/infiniflow/ragflow/pull/7187):

1. Fixes bug #7403 - `redis.persistence.enabled` missing from
`helm/values.yaml` causes helm error:

[ERROR] templates/: template: ragflow/templates/redis.yaml:55:24:
executing "ragflow/templates/redis.yaml" at
<.Values.redis.persistence.enabled>: nil pointer evaluating interface
{}.enabled

2. Fixes regression: reverts hardcoded redis.storage.capacity value back
to using variable `redis.storage.capacity` from `helm/values.yaml`.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

helm/templates/redis.yaml

@@ -76,7 +76,7 @@ spec:
         {{- end }}
         resources:
           requests:
-            storage: 8Gi
+            storage: {{ .Values.redis.storage.capacity }}
   {{- end }}
 ---
 apiVersion: v1

helm/values.yaml

@@ -133,6 +133,8 @@ redis:
   storage:
     className:
     capacity: 5Gi
+  persistence:
+    enabled: true
   deployment:
     strategy:
     resources: