Cannot login to WordPress admin after update

Upgrade your PHP version to 7.2. If that gives you access, update all themees/plugins and WP to latest versions. (PHP version updates done through your hosting Control Panel. Your hosting place should have help docs on how to set your PHP version.)

Test those upgrades, then upgrade PHP to version 8.x.

There are major security issues with PHP 5.x. You should strive to keep everything current. Updating to 7.2 first should help you regain access, then you can finish upgrading to latest everything.