[12.x] prefer stable stability (#6548)

This feels as a leftover from the dev branch. But if I'm wrong, feel free to close this PR.

composer.json

@@ -53,9 +53,6 @@
         ]
     },
     "extra": {
-        "branch-alias": {
-            "dev-master": "12.x-dev"
-        },
         "laravel": {
             "dont-discover": []
         }
@@ -69,6 +66,6 @@
             "php-http/discovery": true
         }
     },
-    "minimum-stability": "dev",
+    "minimum-stability": "stable",
     "prefer-stable": true
 }