CI: Test on Ruby 3.0 instead of 2.x

Ruby 2.x is very old, and newer bundlers don't work with it.

Author: mamhoff

Diff for: .github/workflows/ci.yml

@@ -7,19 +7,19 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - ruby: 2.5
+          - ruby: 3.0
             gemfile: "gemfiles/Gemfile.rails-6.1-sprockets-3"
-          - ruby: 2.5
+          - ruby: 3.0
             gemfile: "gemfiles/Gemfile.rails-6.1-sprockets-4"
 
-          - ruby: 2.7
+          - ruby: 3.0
             gemfile: "gemfiles/Gemfile.rails-7.0-sprockets-3"
-          - ruby: 2.7
+          - ruby: 3.0
             gemfile: "gemfiles/Gemfile.rails-7.0-sprockets-4"
 
-          - ruby: 2.7
+          - ruby: 3.0
             gemfile: "gemfiles/Gemfile.rails-7.1-sprockets-3"
-          - ruby: 2.7
+          - ruby: 3.0
             gemfile: "gemfiles/Gemfile.rails-7.1-sprockets-4"
 
           - ruby: 3.1