Drop support for PHP 7.2, upgrade to PHPUnit 9

[osma]

Support for PHP 8.0 was added in PR #1266, released as part of Skosmos 2.14. Now Skosmos supports four versions of PHP: 7.2, 7.3, 7.4 and 8.0. (There are still problems with PHP 8.1, see #1274)

The next logical step would be to drop support for PHP 7.2, which has been in EOL status by upstream PHP since late 2020, although it is still supported by distributions such as Ubuntu 18.04. Dropping PHP 7.2 would allow upgrading PHPUnit to version 9, which requires at least PHP 7.3. Probably also other PHP libraries are dropping support for PHP 7.2 and we need to do the same in order to be able to upgrade to newer versions.