Upgrading to 3.6 Impossible With PHP 8 Installed
So probably the best thing you can be doing is doing your updates locally, testing them, and then pushing them to the production environment.
You can read more about it in the Updating Craft CMS without Headaches article.
You still have the opportunity to do this here... get a local dev environment set up, pull down your code and database, then get it up and running on a PHP version such as PHP 7.4 that will work with older versions of Craft as well as Craft 3.6.
Then do the update locally, and deploy the update to your production server.