[redis] Bump Redis image version for security fixes (#1580)

This patch updates the RedisFailover Helm template to use a newer,
secure Redis version (8.2.0). This addresses known security issues in
the previous Redis version and ensures safer deployments.

```release-note
[redis] Upgrade Redis to a secure version (8.2.0) to fix security vulnerabilities.
```

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## What this PR does


### Release note

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Redis deployments can now specify the container image via a new
configurable value (default: redis:8.2.0), allowing easy override of the
Redis image used.
* **Schema**
* Values schema and resource definition schemas updated to include and
validate the new image setting.
* **Documentation**
  * README updated to document the new image parameter.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/apps/redis/templates/redisfailover.yaml

@@ -27,6 +27,7 @@ spec:
     replicas: 3
     resources: {{- include "cozy-lib.resources.defaultingSanitize" (list .Values.resourcesPreset .Values.resources $) | nindent 6 }}
   redis:
+    image: "redis:8.2.0"
     resources: {{- include "cozy-lib.resources.defaultingSanitize" (list .Values.resourcesPreset .Values.resources $) | nindent 6 }}
     replicas: {{ .Values.replicas }}
     {{- with .Values.size }}