switch to gem dot coop

[kayiwa]

Let's switch to gem.coop.

This is them: https://gem.coop/

Context: https://www.heise.de/en/news/Who-owns-an-open-source-project-RubyGems-threatens-to-split-10685184.html

[coveralls]

coverage: 65.058%. remained the same
when pulling 3e5b60d on switch_gem_servers
into 969ba26 on main.

[sandbergja]

Thanks, @kayiwa ! 🦒