Fixed TreeBuilder::root() symfony 4.3 deprecation

[Doqnach]

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

[qpautrat]

Hello @Doqnach 👋 ,
Sorry to be so late.
Thank you for your PR.

Since sensio/framework-extra-bundle in ^5.4 accepts symfony/config ^3.4 it means your change implies we break the ^3.4 API because getRootNode function appears only from 4.2.

So we need to bump the major version and specify in composer.json the minimum version required for symfony/config.

What do you think ?

[Doqnach]

For example:

• Fix deprecation for symfony/config 4.2+ symfony/maker-bundle#324
• https://github.com/doctrine/DoctrineBundle/pull/853/files

$treeBuilder = new TreeBuilder('some-root');

if (\method_exists($treeBuilder, 'getRootNode')) {
    $rootNode = $treeBuilder->getRootNode();
} else {
    // BC layer for symfony/config 4.1 and older
    $rootNode = $treeBuilder->root();
}

and then fix this behaviour with next major version, dropping support for older symfony/config.

[qpautrat]

Perfect, fine by me :)

[Doqnach]

I will try to update the PR soon [tm]

[Doqnach]

Updated the PR with the above suggested change.

[qpautrat]

Much appreciated thank you !