Add Ruby 3 build

Closes #84 (closed)

I had to touch many of the initializers, and I don't understand all probes entirely or how to best test them. So this is a bit of a risky change.

But if we cut a new version from it, we could first test it out in Omnibus in a branch? It will be easier to test there with all subsystems up and running and actual data being available.

I also had to update Sinatra to 2.1 to add Ruby 3 support.