update to FFmpeg 6.0

**Is your feature request related to a problem? Please describe.**
I'm using this awesome repo to normalize audio files in my project, and I've encountered a bug in FFmpeg 5.0 where the loudnorm filter does not seem to work for short audio files (about 3 seconds).
Here is the related [issue](https://github.com/slhck/ffmpeg-normalize/issues/87).

**Describe the solution you'd like**
This issue has been fixed in FFmpeg v6.0, so it would be great if we could update FFmpeg to 6.0.

**Describe alternatives you've considered**
I've checked the [Custom Build Guide](https://ffmpegwasm.netlify.app/docs/contribution/core#custom-build--reduce-build-size) but haven't found how to bump the FFmpeg version. Does it work if I just change the FFMPEG_VERSION value?