ffmpeg minimum version check for error handling for VAD failures

I did an OS upgrade on my Synology NAS. This overrode something in my path settings such that ffmpeg4, rather than ffmpeg7, was the default binary aliased to ffmpeg. That's not your problem, and I got it solved. But until I did, every attempt to use ffmpeg gave me this error

https://github.com/smacke/ffsubsync/blob/bafd232a38769951c14c05669a69b2e8c150e1ae/ffsubsync/speech_transformers.py#L430-L434

The error is a reasonable one in hindsight, but it took me a while to figure out that the problem was actually downstream with the ffmpeg binary, not ffsubsync.

My sense is that if there's a minimum ffmpeg version that supports the VAD stuff, then it'd probably be good to either, in advance of running the script or in response to this error, verifying that the ffmpeg binary meets the minimum version requirement. I'd probably insert the check into the ffmpeg_bin_path function here

https://github.com/smacke/ffsubsync/blob/bafd232a38769951c14c05669a69b2e8c150e1ae/ffsubsync/ffmpeg_utils.py#L69

Thanks. ffsubsync is great!

Start in ffsubsync/ffmpeg_utils.py at the ffmpeg_bin_path function and review the VAD failure handling in ffsubsync/speech_transformers.py around lines 430-434. Determine the minimum ffmpeg version required for the VAD functionality and make an unsupported binary produce a clear diagnostic before or during processing. Done means users can distinguish an outdated ffmpeg binary from an ffsubsync error.