Test on Ruby 3.1

Author: horgh

.github/workflows/rubocop.yml

@@ -7,6 +7,6 @@ jobs:
       - uses: actions/checkout@v2
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: '3.0'
+          ruby-version: 3.1
       - run: bundle install
       - run: bundle exec rake -t rubocop

.github/workflows/test.yml

@@ -13,6 +13,7 @@ jobs:
             2.6,
             2.7,
             '3.0',
+            3.1,
             jruby,
           ]
         exclude: