Allow 6.x version of justinrainbow/json-schema

As it's a dev dependency we could consider bumping the lockfile version to 6.x in 11.2 (while still allowing downgrades to 5.x), similar to us upgrading from PHPStan 1 to 2.

I believe this is also blocking composer to be updated past 2.8.7

What would be the issue with allowing either on earlier Drupal versions? I'm trying to use 6.x in a module now and this would mean we'd basically have to wait until 11.1 is EOL to ship these features safely, right? I imagine Experience Builder would have the same issue...