Update sinatra to version 2.2.0

Fixes https://gitlab.com/gitlab-org/gitlab-exporter/-/issues/89

There's a vulnerability that I believe isn't exploitable in gitlab-exporter, though I'm admittedly not familiar with this application. This bumps the version to use the patched gem.

Diff https://my.diffend.io/gems/sinatra/2.1.0/2.2.0