Regenerate existing previews with new dimensions

[gas85]

You do not need to write any script. Simply:

@gas85

To “reset” the preview cache

1. Stop Webserver (apache / nginx)
2. Moved the appdata_instaceName/preview folder outside the appdata folder. You can just remove it if you are confident.
3. Run sudo -u www-data /usr/bin/php /var/www/nextcloud/occ files:scan-app-data to fix database
4. Start Webserver and Test everything works
5. Finish up (delete moved folder, etc).
6. Run pregenerate command afterwards.