FAB should have a shadow at elevation level 3

### Current behaviour
When in M3, the shadow of the FAB is at level 6.

### Expected behaviour
According to [M3 spec](https://m3.material.io/components/floating-action-button/specs), the FAB should have a shadow of level 3. I reported this for the AnimatedFAB back in #3805 and it seems to only have been fixed for the Animated version.

### Your Environment

| software | version
| --------------------- | -------
| react-native | 0.71.8
| react-native-paper | 0.5.8

该 issue 提到了 FAB、AnimatedFAB issue #3805 和 M3 规范，但没有指出文件或测试。首先定位 FAB 的 elevation 实现，并将其与 AnimatedFAB 的修复进行比较，然后根据链接的规范验证 M3 FAB 的阴影是否为 level 3。

由索引模型根据 Issue 内容生成。