There's a merged pull request on the void-packages repository

mkvtoolnix: update to 76.0
https://github.com/void-linux/void-packages/pull/44044

Description:
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#### Testing the changes
- I tested the changes in this PR: **briefly**

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Sticking with Qt5 since `QMediaPlayer` used by `mkvtoolnix-gui` is broken on Qt 6.5.0 (on all platforms, not just Windows as [QTBUG-111951](https://bugreports.qt.io/browse/QTBUG-111951) incorrectly suggests).