MAGETWO-69256: Revert minimum stability to stable, tasks #4359 #9672

 - Merge Pull Request #9672 from ktomk/magento2:patch/composer-minimum-stability-4359
 - Merged commits:
   1. a141ce4
   2. efd4708

Author: Oleksii Korshenko

composer.json

@@ -257,6 +257,5 @@
             "Magento\\TestFramework\\Utility\\": "dev/tests/static/framework/Magento/TestFramework/Utility/"
         }
     },
-    "minimum-stability": "alpha",
     "prefer-stable": true
 }