Add support for symfony 6

[VincentLanglet]

Subject

Symfony 6 is supported by FriendsOfSymfony/FOSCKEditorBundle#239

Changelog

### Added
- Support for Symfony 6

[VincentLanglet]

Now blocked by knplabs/knp-markdown-bundle

[VincentLanglet]

Need to follow KnpLabs/KnpMarkdownBundle#111

[jordisala1991]

It is really strange to have removed all the code related to ckeditor or markdown, but still have the dependencies, right?

[jordisala1991]

Looking at: https://github.com/KnpLabs/KnpMarkdownBundle/releases

Even though we could restore support directly here for markdown, IMO this dependency is not well maintained.
I would vote to remove everything about markdown and have a documentation about how to add it in your own project.

[VincentLanglet]

It is really strange to have removed all the code related to ckeditor or markdown, but still have the dependencies, right?

Indeed, I removed it.

I kept the ckeditor one since we plan on restoring the support.