Fix for upcoming 10.9 release

[gnattu]

This PR:

• Targets macOS 12 because dotnet 8 officially requires this version to run. This will allow us to use new APIs in the future. Our FFmpeg also relies on modern OS APIs to perform certain hardware acceleration operations.
• Changes the banner to show "2024."
• Fixes the command line for 10.9. It now specifies the ffmpeg path using the --ffmpeg CLI parameter. This would make it impossible for our users to specify a custom ffmpeg binary, and if that is a supported use case, then an additional setting for this in this app is required.

[anthonylavado]

It all looks good to me. As for ffmpeg, I feel like people might have used the homebrew one in the past. Now that we're bundling our own with significant improvements, I don't anticipate it being an issue.

If it is, we'll cross that bridge when we come to it.