ProForm 报错 React does not recognize the `showCount` prop on a DOM element

### 🐛 bug 描述
ProForm 报错 React does not recognize the `showCount` prop on a DOM element

### 📷 复现步骤
1. ProForm 设置readonly、initialValues
2. ProFormTextArea 添加 showCount，如：fieldProps={{ maxLength: 500, showCount: true }}
3. 控制台报错了

### 🏞 期望结果
不要报错
### 💻 复现代码
```javascript

```

### © 版本信息

- ProComponents 版本：最新版本
- umi 版本：4.2.15
- 浏览器环境：谷歌
- 开发环境：mac OS

### 🚑 其他信息

Reproduce the warning with the provided ProForm, ProFormTextArea, readonly, initialValues, and fieldProps example. Start by tracing ProFormTextArea's readonly rendering and how showCount reaches the DOM. Done means the example no longer logs the React prop warning while preserving the requested textarea behavior.

Written by the indexing model from the issue text.