Currently, jump to next/previous strip use page up and page down,
respectively. This seems backwards, as up should go back and down
should go forward, same as in a text editor.
This commit swaps the shortcuts for Blender default and
industry-compatible keymaps.
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This behaviour was introduced at the same time as the operator in c2f9cfb40a.
Pull Request: https://projects.blender.org/blender/blender/pulls/138339