[String] Add conflict for translation-contracts v3

Author: chalasr

src/Symfony/Component/String/composer.json

@@ -29,6 +29,9 @@
         "symfony/translation-contracts": "^1.1|^2",
         "symfony/var-exporter": "^4.4|^5.0"
     },
+    "conflict": {
+        "symfony/translation-contracts": ">=3.0"
+    },
     "autoload": {
         "psr-4": { "Symfony\\Component\\String\\": "" },
         "files": [ "Resources/functions.php" ],