build: allow installation of symfony/string version 7

Author: JaZo

composer.json

@@ -28,7 +28,7 @@
         "php-http/mock-client": "^1.6",
         "psr/http-factory": "^1.0",
         "psr/http-message": "^1.0|^2.0",
-        "symfony/string": "^5.4|^6.0"
+        "symfony/string": "^5.4|^6.0|^7.0"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^3.16",