Bump min Ruby version to 3.1

Rails 7.2 isn't compatible with Ruby 3.0, and 3.0 was made EOL several
months ago.

.github/workflows/main.yml

@@ -8,12 +8,8 @@ jobs:
 
     strategy:
       matrix:
-        ruby-version: ['3.0', '3.3']
+        ruby-version: ['3.1', '3.3']
         gemfile: ['Gemfile', 'Gemfile.rails71']
-        exclude:
-          # Rails 7.2 requires Ruby 3.1+
-          - ruby-version: '3.0'
-            gemfile: 'Gemfile'
 
     steps:
       - uses: actions/checkout@v4