Replace rubygems.org with gem.coop

Several maintainers of RubyGems created gem.coop as a community-governed
service after a takeover by RubyCentral, pushed by Shopify.

We are moving with the community that reflects our values best.

Gemfile

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-source 'https://rubygems.org'
+source 'https://gem.coop'
 git_source(:github) { |repo_name| "https://github.com/#{repo_name}.git" }
 
 ruby File.read('.ruby-version').chomp

Gemfile.lock

@@ -46,7 +46,7 @@ PATH
     web (0.0.1)
 
 GEM
-  remote: https://rubygems.org/
+  remote: https://gem.coop/
   specs:
     Ascii85 (2.0.1)
     actioncable (7.1.6)