Restrict faraday upper version range

Similar to https://github.com/arthurnn/twirp-ruby/pull/124

The current version range of `faraday` is set to `< 3`. This caused an error in our Twirp API CI/CD pipelines due to (what appears) to be a breaking change that was introduced between versions `2.11.0` and `2.12.0` of `faraday` (released today).

Specifically, it appears to be around an explicit json dependency added in `2.12.0` of the Gem. [2.12.0 release notes](https://github.com/lostisland/faraday/releases/tag/v2.12.0). We resolved the issue by explicitly requiring `2.11.0` of Faraday in our tooling.

I'm not intimately familiar in `faraday`'s release process, so I can't speak to if introducing breaking changes in minor versions is normal for that lib. However, I think it would be good if we could protect consumers of `twirp` Ruby by being a bit more restrictive on the upper bound of the version range.

Locate the twirp-ruby dependency declaration and compare its current Faraday constraint with the reported CI failure between versions 2.11.0 and 2.12.0. Check the Faraday 2.12.0 release notes and reproduce or run the affected CI checks; done means the supported range no longer admits the breaking version and the checks pass.

Written by the indexing model from the issue text.