Remove EOL PHP 7.4 and update default version (conservatively) to 8.0

We're in a little bit of a weird place right now: PHP 7.4 is now EOL, but it's the only non-beta-support version of PHP supported by the latest release of WordPress (https://make.wordpress.org/hosting/handbook/server-environment/#php, https://make.wordpress.org/core/handbook/references/php-compatibility-and-wordpress-versions/, https://core.trac.wordpress.org/ticket/57109#comment:3).  For now, this opts to move our "default" version (for users who don't specify a version) to the conservative PHP 8.0 (the oldest still-supported "beta support" version).

Author: tianon