[release-2.16] fix: missing default values for system settings (#6036)

This is an automated cherry-pick of #6035

/assign JohnNiang

```release-note
修复系统设置未保存导致无法正常注册的问题
```

application/src/main/resources/extensions/system-configurable-configmap.yaml

@@ -6,7 +6,8 @@ data:
   user: |
     {
       "allowRegistration": false,
-      "defaultRole": "",
+      "mustVerifyEmailOnRegistration": false,
+      "defaultRole": "guest",
       "avatarPolicy": "default-policy"
     }
   theme: |