community/cacti: upgrade to 1.2.19 and use php8

• https://www.cacti.net/info/changelog#cacti-1-2-19
• it should be a bit faster
• php7 going to security support in 2 months https://www.php.net/supported-versions.php
• supports php8 since 2.1.17 https://github.com/cacti/cacti/issues/4001