Replace Webpack-Encode with AsssetMapper

[Hanmac]

@VincentLanglet when is Sonata going to drop Symfony 5.4 Support? The AssetMapper might look more interesting than webpack

We're following https://symfony.com/releases#maintained-symfony-branches
so it will be end of the year.

But you can start a PR with AssetMapper if you want.

Originally posted by @VincentLanglet in #482 (comment)

See also #480