Support Symfony 6 (#33)

Hi folkes! Love the lib, use it on one of my projects extensively!

I am in the middle of trying to upgrade my codebases to run on Laravel 9 and because they now based out of Symfony 6, This package is blocking the upgrade, So suggesting support to include the latest version!

All my best 🥇

Co-authored-by: Andreas Frömer <icanhazstring@users.noreply.github.com>

composer.json

@@ -21,7 +21,7 @@
     ],
     "require": {
         "php": ">=7.2",
-        "symfony/process": "^4.4 || ^5.0",
+        "symfony/process": "^4.4 || ^5.0 || ^6.0",
         "ext-iconv": "*",
         "ext-dom": "*",
         "ext-libxml": "*"