The 8.x-1.2 version has a change in composer.json, we do not require the dev version from geshi library, we require the last stable version. If you have a error in composer when updating to this release, please remove the geshifilter module and require it again:
composer remove drupal/geshifilter
composer require drupal/geshifilter
You do not need to uninstall the module, so you will not lose any settings. This is only for people that use composer, if you do not use, it is not necessary any change.