[BUG] dashboard读取环境变量时使用点分割，而非文档提到的下划线分割

## Issue Description

Type: *bug report*

### Describe what happened

dashboard从环境变量中读取配置时（如密码），并未使用文档中提到的下划线分割格式（sentinel_dashboard_auth_password），而是使用点分割（sentinel.dashboard.auth.password）。

### Describe what you expected to happen

文档上说读取环境变量时使用下划线分割。

### How to reproduce it (as minimally and precisely as possible)

1. 创建sentinel容器（可以使用`docker.io/magaical/sentinel-dashboard:v1.8.8`），并将密码作为环境变量（`sentinel_dashboard_auth_password` / `sentinel.dashboard.auth.password`）传入容器
2. 使用自定义的密码登录dashboard
3. 只有`sentinel.dashboard.auth.password`能登录成功

### Tell us your environment

Fedora Linux 42.20250807.0 (Silverblue)
podman 5.5.2

### Anything else we need to know?

https://sentinelguard.io/zh-cn/docs/dashboard.html

https://github.com/alibaba/Sentinel/blob/5473ad7b3e3ce3248c82590dabee9bb93c3894b6/sentinel-dashboard/Sentinel_Dashboard_Feature.md?plain=1#L54

https://github.com/alibaba/Sentinel/blob/5473ad7b3e3ce3248c82590dabee9bb93c3894b6/sentinel-dashboard/src/main/java/com/alibaba/csp/sentinel/dashboard/config/DashboardConfig.java#L48

https://github.com/alibaba/Sentinel/blob/5473ad7b3e3ce3248c82590dabee9bb93c3894b6/sentinel-dashboard/src/main/java/com/alibaba/csp/sentinel/dashboard/config/DashboardConfig.java#L117-L119