Update environment.xml in branches to reflect minimum required PHP version

https://github.com/catalyst/moodle-auth_saml2/blob/MOODLE_404_STABLE/environment.xml

The branches MOODLE_404_STABLE and MOODLE_405_STABLE still require only PHP 7.4 which isn't right anymore because newer simplesamlphp versions require PHP 8.1

You could also still add newer simplesamlphp versions to Moodle 4.1 if you just require PHP 8.1 there and say so in the readme...