Upgrade to Ruby 3.2, Sinatra 3.0, Rack 2.2

Summary:
As webrick isn't included by default to Ruby 3.x standard library,
a webserver needs to be picked. Thin will do.

Sinatra isn't ready for Rack 3.

Test Plan:
Container built and tested for ci.nasqueron.org (multi-branch)
and cd.nasqueron.org (regular setup).

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2897