Bump minimum Commonmark requirement

Author: fabpot

composer.json

@@ -22,7 +22,7 @@
     "require-dev": {
         "symfony/phpunit-bridge": "^6.4|^7.0",
         "erusev/parsedown": "dev-master as 1.x-dev",
-        "league/commonmark": "^1.0|^2.0",
+        "league/commonmark": "^2.7",
         "league/html-to-markdown": "^4.8|^5.0",
         "michelf/php-markdown": "^1.8|^2.0"
     },