Add PHP 8.0 support

Author: hajekj

composer.json

@@ -22,7 +22,7 @@
         "sso"
     ],
     "require": {
-        "php": "^5.6|^7.0",
+        "php": "^5.6|^7.0|^8.0",
         "league/oauth2-client": "~2.0",
         "firebase/php-jwt": "~3.0||~4.0||~5.0"
     },