UnderDogMiller
Ubuntu 20.04 is still good until 2025, the update notification you get to upgrade to 22.04, you don't need to do now, you can still do normal updates by running sudo apt update && sudo apt upgrade
The main issue you are having is the PHP version and modules, just make sure you have installed
if you are currently on PHP 7.4, make sure to install PHP 8.0 or 8.1 for osTicket v17.2
Here is command for PHP 8.0:
sudo apt install php8.0 php8.0-mysql php8.0-mbstring php8.0-gd php8.0-intl php8.0-apcu php8.0-imap php8.0-xml php8.0-zip
for PHP 8.1:
sudo apt install php8.1 php8.1-mysql php8.1-mbstring php8.1-gd php8.1-intl php8.1-apcu php8.1-imap php8.1-xml php8.1-zip
Then you need to enable your PHP version: sudo a2enmod php8.0 or php8.1
Then restart the service: sudo systemctl restart apache2
I wrote a PHP upgrade post it's little older but the process kind of similar: https://howtohelpdesk.com/how-to-upgrade-to-php-7-4-on-ubuntu/