libobs: Fence off unnecessary code due to FFmpeg v6.1 changes

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Fixes for using FFmpeg 6.1 due to deprecations. Uses `#if` macros to
allow builds for using older versions of FFmpeg.

This commit prevents obs from using the "fenced" code if using FFmpeg
6.1, since in FFmpeg commit [1] the "side_data" is added to
`AVCodecParameters`, and therefore the existing/following
`avcodec_parameters_copy(...)` will account for the metadata.

[1]: avcodec/codec_par: add side data to AVCodecParameters
https://github.com/FFmpeg/FFmpeg/commit/21d7cc6fa9a26e94965fa71b25655d07568450fe

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