Allow Symfony 6

[Slamdunk]

Fixes #416

[Slamdunk]

Regarding the BC Break reported by CI: I think BrowscapPHP\Helper\Filesystem should be @internal

[Slamdunk]

Ping @asgrim

[asgrim]

@Slamdunk thanks - we'll take this further upstream, even latest BCC tool falters here, I can't think why at the moment (see also #418 )

[asgrim]

Yeah, BC breaks are correct here, because we use extends for both Symfony Commands and Filesystem as you found, then upstream BC breaks are also our own inherited BC breaks. I'm closing this in favour of #418 where a BC break here is necessary to make our own classes @internal to avoid this going forward. Thank you!