Faraday 2 support (update CI and dependencies)

[iMacTia]

Summary

This PR refreshes outdated development dependencies and more notably sets Ruby 3.0 as the minimum Ruby version (this will likely warrant a new major release). The reason is that Ruby 3.0 is nearing EOL already, and Faraday 2.0 does not support Ruby 2.7 or earlier.

CI has also been improved to test the correct Ruby versions, as well as adding a dimension to test both Faraday 1.x and 2.x

[iMacTia]

@bhaberer Would you like to review PRs before merging or do you prefer a more hands-off approach?
I understand you're not using the library anymore, but it's still under your name so happy either way 🙂

[iMacTia]

Btw I wasn't expecting all tests to pass with Faraday 2.0 🤔
I've reached out to @rgaufman to understand what issues he's running into when using it with Faraday 2

[rgaufman]

Sorry for the late reply, I just assumed because of the documentation it won't work, I never actually tried it! - I just did and so far it seems to be working, thank you!

[bhaberer]

LGTM

[iMacTia]

Glad to hear the current version is working already, we can still merge this and might not even need to release a new major version because bundler should be smart enough to just ignore it if you're running an older Ruby 👍

[bhaberer]

@iMacTia @rgaufman FYI, will probably tag head as v0.5.0 and push a new version to rubygems tonight/this weekend unless either of you object :) Thanks again for the help!

[rgaufman]

No objections from me and much appreciated :)