resolves #16634

I faced the same issue.
ODROID-N2+ comes with Ubuntu 20.04.
I agree that the straight way to solve the issue is to install newer ffmpeg using apt
Still, I don't see a reason a to keep a keywords which should be covered by macro.

Pull Request Readiness Checklist

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• I agree to contribute to the project under Apache 2 License.
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  Patch to opencv_extra has the same branch name.
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