Revert "fix: branch-alias for 2.6 (#4602)"

This reverts commit 57ff81f.

Author: soyuka

composer.json

@@ -139,7 +139,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-main": "2.6.x-dev"
+            "dev-main": "2.7.x-dev"
         },
         "symfony": {
             "require": "^3.4 || ^4.4 || ^5.1 || ^6.0"