* Add the PHP PPA.
* Specify the version when invoking the php CLI.
* Specify the version in package names.
* Update paths to 8.0 (using a variable in the setup scripts).
* Update z-push's php-xsl dependency to php8.0-xml.
* php-json is now built-into PHP.
Although PHP 8.1 is the stock version in Ubuntu 22.04, it's not supported by Nextcloud yet, and it likely will never be supported by the the version of Nextcloud that succeeds the last version of Nextcloud that supports PHP 7.2, and we have to install the next version so that an upgrade is permitted, so skipping to PHP 8.1 may not be easily possible.
* drop the ondrej/php PPA since PHP 7.x is available directly from Ubuntu 18.04
* intall PHP 7.2 which is just the "php" package in Ubuntu 18.04
* some package names changed, some unnecessary packages are no longer provided
* update paths
* Install PHP7 via a PPA, enable unattended upgrades for the PPA, and switch all of our PHP configuration to the PHP7 install.
* Keep installing PHP5 for ownCloud/Nextcloud packages because we need it to possibly run transitional updates to ownCloud/Nextcloud versions less than 12. But replace PHP5 packages with PHP7 packages elsewhere.
* Update to Nextcloud 12 which requires PHP7, with a transitional upgrade to Nextcloud 11.0.3.
* Disable TLS cert validation by Roundcube when connecting to localhost IMAP and SMTP. Validation became the default in PHP7 but we don't necessarily have a (non-self-)signed certificate and it definitely isn't valid for the IP address 127.0.0.1.
Merges #1140
