Mark the toolkit not PHP 8.0 ready

Author: pitbulk

composer.json

@@ -15,7 +15,7 @@
         "source": "https://github.com/onelogin/php-saml/"
     },
     "require": {
-        "php": ">=5.4",
+        "php": ">=5.4 <8.0",
         "robrichards/xmlseclibs": ">=3.1.1"
     },
     "require-dev": {