wip: drop support for Rack < 3

[zzak]

Fixes #50565

One thing I'd like to figure out in this PR is rack/rack#2137, so still investigating. This is more just an FYI, if folks would like to help.

[Earlopain]

Well, that deprecation for the status code was reverted for 3.1 but that doesn't really help when rails is testing against main. Something like #52097 needs to be done regardless. I don't think it was finished though, seems to have missed a few instances (apart from docs).

[ioquatix]

I've been discussing this with @zzak and I think we'd be okay with dropping the deprecation warnings from 3.2 and re-introducing them in Rack 3.3, that gives us 12 months to sort out Rails in terms of what's running on head, and ~24 months for what's actually released, before we start issuing deprecation warnings.

[zzak]

There is also rails/buildkite-config/#110 to update the CI matrix