BottomNavigation.Bar applies wrong fontWeight to active label

### Current behaviour
The `BottomNavigation.Bar` applies the same `fontWeight` to all labels (active and inactive) as seen [in the implementation](https://github.com/callstack/react-native-paper/blob/ff0df5454eb13d6e8e2d8f1c87c0bca8bb3635f0/src/components/BottomNavigation/BottomNavigationBar.tsx#L850) but according to the [MD3 spec](https://m3.material.io/components/navigation-bar/specs) the weight for the active label should be bold (i.e. 700).

### Expected behaviour
Active label should be bold.

### How to reproduce?
Use `BottomNavigation.Bar` (e.g. together with react-navigation).

### Preview

### What have you tried so far?

Read the implementation, which clearly is wrong.

### Your Environment

| software | version
| --------------------- | -------
| ios | x
| android | x
| react-native | x.x.x
| react-native-paper | 5.14.5
| node | x.x.x
| npm or yarn | x.x.x
| expo sdk | x.x.x

从 src/components/BottomNavigation/BottomNavigationBar.tsx 中 issue 链接的实现开始，并将活动和非活动标签的样式与 MD3 导航栏规范进行比较。在 iOS 和 Android 上使用 BottomNavigation.Bar 时验证该组件；当活动标签使用 fontWeight 700，而非活动标签保持其预期的字重时，即表示完成。

由索引模型根据 Issue 内容生成。