Currently, we need to release a new version of the migration plugin for each new WordPress version. This is not ideal, but “supporting” WP versions when we haven’t actually tested them is also not ideal. Open to ideas and suggestions there.
In the meantime there is a way to override the WP version check by adding a line of code in your theme’s functions.php file or a file under wp-content/mu-plugins. See: GitHub - ClassicPress/ClassicPress-Migration-Plugin: Switch your WordPress installation to ClassicPress.