Add support for Symfony 7

[stof]

Closes #347

Due to the requirement to add the native return type in the Configuration class and the DumpCommand class which were neither final nor internal, this should be released as a new major version to respect semver.
All the void return type I'm adding as native return types are actually optional in Symfony 7. They will become required only in Symfony 8. As adding them also requires a major version, I'm adding them at the same time (so that Symfony 8 does not require a new bump of the major version).

I also took the opportunity to mark classes as @final or @internal to reduce the BC API of the bundle for the future. Note that for now, the command and controller are only marked as @final but I'm wondering whether they could be marked as @internal instead if they are only meant to be initialized by the service definition of this bundle.

[stof]

@monteiro when do you expect to make the release ?

[monteiro]

Today or tomorrow max.