fix symfony 4.3 deprecation about root nodes

"The "Symfony\Component\Config\Definition\Builder\TreeBuilder::root()" method called for the "cmf_routing" configuration is deprecated since Symfony 4.3, pass the root name to the constructor instead."

Author: killerwolf

src/DependencyInjection/Configuration.php

@@ -33,8 +33,13 @@ class Configuration implements ConfigurationInterface
      */
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
-        $root = $treeBuilder->root('cmf_routing');
+        $treeBuilder = new TreeBuilder('cmf_routing');
+        if (method_exists($treeBuilder, 'getRootNode')) {
+            $root = $treeBuilder->getRootNode();
+        } else {
+            // BC layer for symfony/config 4.1 and older
+            $root = $treeBuilder->root('cmf_routing');
+        }
 
         $this->addChainSection($root);
         $this->addDynamicSection($root);