PHP8 compat

[rfair404]

In a php8 environment, the following error is being thrown: Function libxml_disable_entity_loader() is deprecated

Based on the stack trace:

 in OneLogin\Saml2\Utils::loadXML called at /srv/www/custom-content/plugins/wp-simple-saml/vendor/onelogin/php-saml/src/Saml2/IdPMetadataParser.php (115)
 in OneLogin\Saml2\IdPMetadataParser::parseXML called at /srv/www/custom-content/plugins/wp-simple-saml/inc/admin/namespace.php (58)
 in HumanMade\SimpleSaml\Admin\get_config called at /srv/www/wp/wp-includes/class-wp-hook.php (307)
 in WP_Hook::apply_filters called at /srv/www/wp/wp-includes/plugin.php (189)
 in apply_filters called at /srv/www/custom-content/plugins/wp-simple-saml/inc/namespace.php (201)
 in HumanMade\SimpleSaml\instance called at /srv/www/custom-content/plugins/wp-simple-saml/inc/namespace.php (165)
 in HumanMade\SimpleSaml\authenticate_with_sso called at /srv/www/wp/wp-includes/class-wp-hook.php (309)
 in WP_Hook::apply_filters called at /srv/www/wp/wp-includes/class-wp-hook.php (331)
 in WP_Hook::do_action called at /srv/www/wp/wp-includes/plugin.php (522)
 in do_action_ref_array called at /srv/www/wp/wp-includes/user.php (67)
 in wp_signon called at /srv/www/wp/wp-login.php (1211)

Updates onelogin/php-saml to use ^4.0, which includes support for php >8.

Reference SAML-Toolkits/php-saml#453

[rfair404]

@tfrommen how can I get this reviewed and moving?

[roborourke]

I think we'll need a new major version bump for this but the best outcome might be to remove the bundled version completely, and set the version constraint to ^3 || ^4. Sorry this has taken so long to get to.