FFmpeg v8.0.0 full

## 4.0.0

*   Upgraded to FFmpeg `v8.0.0`, which includes numerous updates like new encoders and decoders and Vulkan compute-based codecs.
*   Updated the Android `ffmpeg-kit-full-gpl` dependency.
*   Raised the minimum macOS deployment target from `10.14` to `10.15`.
*   Raised the minimum iOS deployment target from `12.0` to `13.0`.
*   Corrected the unzip destination path in iOS and macOS setup scripts.
*   Updated `CHANGELOG.md`, `README.md`, and `pubspec.yaml` with the new version.