`enable_admin_ui` is not rendered when using external etcd

## Description

In `templates/configmap.yaml`, the `enable_admin_ui` setting is inside a `{{- if .Values.etcd.enabled }}` block:

```yaml
admin:
{{- if .Values.etcd.enabled }}
enable_admin_ui: {{ .Values.apisix.admin.enable_admin_ui }}
{{- end }}
```

When you use an external etcd (`etcd.enabled: false` with `externalEtcd` configured), this line never makes it into the rendered config. APISIX defaults to `enable_admin_ui: true`, so the dashboard ends up exposed on port 9180 even though you explicitly set `enable_admin_ui: false` in your values.

## Steps to reproduce

1. Deploy with external etcd:
```yaml
etcd:
enabled: false
externalEtcd:
host:
- "https://my-etcd:2379"
apisix:
admin:
enabled: true
enable_admin_ui: false
```

2. Run `helm template` — the rendered configmap has no `enable_admin_ui` line under `deployment.admin`.

3. Hit `http://:9180/ui/` — dashboard is accessible.

## Expected

`enable_admin_ui: false` should be rendered regardless of how etcd is deployed. The setting controls the Admin API server behavior, not etcd connectivity.

## Suggested fix

Remove the `etcd.enabled` guard:

```yaml
admin:
enable_admin_ui: {{ .Values.apisix.admin.enable_admin_ui }}
```

## Environment

- Chart: apisix 2.16.0
- APISIX: 3.17.0
- Helm: 3.x

Open templates/configmap.yaml and inspect the deployment.admin section around the enable_admin_ui setting. Render the chart with etcd.enabled: false, externalEtcd configured, and apisix.admin.enable_admin_ui: false using helm template. Done means the rendered ConfigMap contains enable_admin_ui: false under deployment.admin for external etcd.

Written by the indexing model from the issue text.