Allow Symfony 6.0+

[tchapi]

This PR bumps the dependencies in composer.json to allow for Symfony 6.0+

[tchapi]

I guess we're going to do this:

• Allow Symfony 6.0 (this PR) while not touching anything else
• Tag a minor release since it's not a breaking change per se. That will be 3.1.0
• Later on (TBD), remove Symfony < 4 support

If @webda2l is OK with this, I'll merge this afternoon (CET) and tag the release :)

[tchapi]

See https://github.com/a2lix/TranslationFormBundle/releases/tag/3.1.0 🎉