Add tests for PHP 7.3 or drop outdated PHP Versions?

PHP 7.3 is supported but not tested in the CI.
https://github.com/geocoder-php/provider-integration-tests/blob/master/composer.json#L19

This should be changed in the CI.
Or can the support for PHP < 8.0 or 8.1 be dropped?
I know that the main repo still supports PHP 8.0 for the moment.
