[BUG] Slide.__background_color is broken on latest manimce

### Description

Right now, manim-slides calls `ManimColor.hex` in `__background_color`, which no longer exists in the latest manim community edition. It needs to be changed to use `ManimColor.to_hex()` instead.

### Version

4.16.0; also present in main.

### Platform

Windows. The issue is platform-independent.

### Screenshots

_No response_

### Additional information

_No response_