Check if the SDK is Rack 3 compatible

[st0012]

Rack 3 is on its way and will include multiple breaking changes. We should check if the SDK & its tests need any change for it.

[st0012]

Upgrading to Rack 3 will cause multiple errors in the sentry-rails test suite. But I think the cause is between Rack 3 <-> Rails and have opened an PR for it: rack/rack#1876

Update: The issue is an intentional spec change on Rack and Rails will need to update its implementation for it. So there's nothing we can do about it.

With that being said, both Sinatra and Rails lock their rack at 2.*, so we won't need to actively watch Rack 3's release until they also push a big release for Rack 3 support (which can take months). I'll close this for now.

cc @sl0thentr0py